Exercise progressions don't work for everyone.
What's an exercise progression? A series of progressively harder versions of an exercise. For pushups, it would be something like:
Barbell or Wall Pushups
Feet Elevated Pushups
Feet Elevated Pushups w/hands on med ball(s)
One-Arm Wall Pushups
Feet Elevated One-Arm Pushups
Not for everyone? Sometimes a variation does a bit more than just make the exercise harder. While a 315 pound squat is demonstrably and measurably harder than a 310 pound squat, it's not precisely the same with changing exercises. A one-arm pushup is harder than a pushup, but it also requires some different body positioning. A handstand pushup (the ultimate in "feet elevated") is more like a vertical press than a horizontal press. The exercise hasn't been made more difficult, it's been made different, and that different exercise is inherently harder.
So what happens if you have someone with a shoulder impingement who can't press overhead without injury? Or someone with a spinal loading issue (compressed vertebrae + loading overhead = bad idea)? You can't easily progress them up.
Sometimes you'll often get oddities. I've had clients with knee pain that prevents them from doing a lunge. On the normal progression, something like a Bulgarian split-squat is easier than a dynamic lunge. By any "normal" progression, I'd avoid the split-squat too. However I've found that for knee pain issues, the lack of forward knee tracking on the Bulgarian split-squat means very little knee stress for those clients.
This doesn't mean progressions are meaningless. But it's worth being aware as a trainee or a trainer that everything isn't always going to move along in an orderly fashion for every client.