I'm afraid OP's assumptions are not correct. Its' not that we see, like, 10 move mate patterns. That's nonsense. Those are calculated. And it takes time. Sometimes ten minutes, sometimes 40 minutes! Nobody calculates dozens of moves ahead either. It's total nonsense too. There are forced positions, yes. In such positions sometimes 20 moves ahead can be calculated if these are forced positions, e.g., with mate threats, or material loss. Again, OP's assumptions are inaccurate as to these positions being totally forced. They are not, especially long ones. Even short ones are usually not absolutely forced. However, they are forced in a way that material can be lost, which means certain moves need to be played, otherwise (if material is lost) you can resign on the spot. However, it's not like mate is coming right away. No, that's incorrect. It's the other way round. The defensive side has to hold on to the material no matter what (or it's resignable on the spot - nobody is going to play a piece down other things being equal at high level) which creates kind of forced lines to calculate. As I said, such lines can be 20 moves deep and even more. Here's a situation like that from a real game but it's not something too unusual or especially weird. It's not the longest sequence either but it's 18 moves long and it ends with a mate (White's to move):
[fen "r1b3nr/pp1p1k1p/4ppp1/q2Nb3/8/1B6/PP3PPP/R1BQR1K1 w - - 0 1"]
1. Rxe5 fxe5 2. Qf3+ Ke8 (2... Kg7 3. Bh6+ Nxh6 ( 3... Kxh6 4. Qf8+ Kg5 5. g3 Qd2 6. h4+ Kg4 7. Kg2 Kh5 8. Kh3+-) 4. Qf6+ Kg8 5. Ne7#) 3. Bh6 Nxh6 4. Qf6 Ng8 ( 4... exd5 5. Qxh8+ Ke7 6. Qxe5+ Kf8 7. Re1 Qd2 8. Qe8+ Kg7 9. Re7+ Kf6 10. h4 Qc1+ 11. Re1+-) 5. Qxh8 Kf7 6. Qxh7+ Kf8 7. Bc2 Qxd5 8. Bxg6 Nh6 9. Qxh6+ Ke7 10. Qg7+ Kd6 11. Qf8+ Kc6 12. Rc1+ Kb5 13. a4+ Ka6 (13... Kxa4 14. Qa3+ Kb5 15. Bd3+ +-) 14. Qb4 Qa5 15. Bd3+ b5 16. axb5+ Kb6 17. Qd6+ Kb7 18. Be4#
White had to calculate 18 moves ahead here till mate. Of course, an engine will see right off the bat that Black's losing and will focus only on prolonging the game. Humans don't play like that. As I said it is as good as resigning on the spot. What good is it prolonging the game if you consider it dead lost and await only a mate? So, Black held on as hard as it could to stay in the game. However White had calculated 18 moves ahead till mate and won this game. This game is a puzzle now for highly professional players. It's extremely forcing. Yet, it's not absolutely forcing like the OP wanted. Absolutely forcing very long lines can be found in artificial made-up puzzles or they can be generated by the computer. Made-up puzzles are often absolutely forcing, e.g., mate in 8, 12 or 16 moves.
The forced mating position is not absolutely forced but Black may lose material or continue to hang on at the risk of being mated. This is not the longest example the OP wanted (longer and more complicated forced examples from real games exist I can assure you). It's a clarification of misunderstanding. The above diagram is a mere example of a forced play with almost all moves being checks hardly leaving any leeway for Black. That's why it is short in terms of branching out with most subvariations leading to Black's fast demise and hence, they are dismissed out of hand (even if they protract the game beyond 25 moves). The engine will play it as if it's all calculated and opponent has, like, Elo 3500, and since it's dead lost the engine will seek only to prolong the game and will happily give away material and whatever just to receive mate later. Even in the dumbest situation when a mate is looming four moves ahead, the engine will sacrifice pieces checking your king, e.g, checking your king with a Queen so that you capture said Queen to drag the situation by one more move. Humans could not care less about such things. Humans aren't going to give away material with other things being equal if there's a chance to fight off the attack and bring the king to safety.
What's more, I don't really know if the player calculated all 18 moves (he probably did) in the diagram above but that's irrelevant because this line is pretty forcing anyway. It means it is not that difficult for a professional to calculate it through in 20-30 minutes.
Bottom line: if the sequence is as long as 20 moves, human analysis is going to be imperfect or it's going to be a made-up puzzle because such puzzles can be absolutely forcing and long. Real game positions are usually not 100% forcing, especially if they are long. Actually a human can also drag the game out. In the diagram above if Black had been told to drag out the game and make sure not to get mated in 25 moves no matter the cost, even at the expense of the Queen, I'm sure Black would have succeeded with the task. But to what end? To make sure the game is really lost but not too fast? Point being, White could have slipped up if the above 18 moves had not been thought out well enough. Black bravely tried every trick in the book to bring the king to a safe haven and keep a relative material parity. But if Black had lost a piece while bringing its king to safety, the game would have been completely lost even with a slight inaccuracy from White. However, if some inaccuracy had been made in the calculation of 18 moves above, Black might have drawn.
Now then, something like mating with a knight and bishop is perceived as forcing but it is not something we calculate with any great depth. That's because the losing side has too many moves, and it's too much of branching out to be calculated. Yet, it's an easy win by following an algorithm, a recipe like a stepladder mate. It's just a more advanced recipe. It's wackiness to think mating with a knight and bishop is some forcing calculation that a player made at some ridiculously long depth of 25 moves ahead. It's never like that. Only computers calculate like that and only when they don't have the endgame tablebase. If they have it, they just retrieve ready solutions. And if I made 10-15 moves exactly perfect according to engine or tablebase while mating with a bishop and knight, it doesn't mean I was calculating 10-15 moves ahead. No, it was just a coincidence. It means I was simply using the technique of mating with a bishop and knight and some of it just happened to be in line with perfect play. That's all there's to it.
