Its a mental thing, they gotta keep up with the big boys, and get bigger.
As you say, they f**k it up most of the time. Perfectly competitive physiques get ruined.
As amateurs, these individuals are bound by weight classes, and usually try to stay just on the high-end of the weight limit, unless they're super-heavies, in which case the need to come in sharp dictates the bw.
Once they turn pro, the need to keep up with the Joneses, makes the guys put on ridiculous amounts of size in a short time frame, whilst when they were amateurs it took 5 years to put on the equivalent.
In the process, they lose sharpness, symmetry suffers, the gut gets distended, weak bodyparts become more prominent.
Great examples of not fixing what aint broke were Labrada and Ray.
Priest is one that put on size gradually, and didnt stuff-up his shape.
Kamali is on the other end of the scale.