Generally, it’s because “growth” resulting from lig work is, more or less, the inner penis being pulled from the body.
Think about it - a guy with a high LOT & high angle of erection generally has tight/short ligs. As he lengthens the ligs, the point his penis “exits” the body gets lowered, freeing up possibly a relatively quick inch of “length.”
Now, once a lot of lig work gets that guy’s LOT down around 6-7, what else can he do? If he continues until his LOT is exactly 6:00, what more length can he expect? Does he think he’s going to actually separate the ligs from the pubis? (that would be a nightmare). He’s not going to get his penile “exit point” down near his asshole (and why would he want to?).
The key, I believe, is to alternate tunica & ligament work, moving your LOT down, then back up again, then down, etc.
After you’ve squeezed that initial quick length out of the ligs, you need to hit the tunica for more length gains. Over time, your LOT will rise again - I know mine did. Then you hit the ligs again.