My theory is that practical effects takes a monumental amount of knowledge and skill and as those people got more and more expensive it was cheaper for the vultures to just hire college grad artists and grind them into the ground than pay the union salaries.
I think it takes the same amount of knowledge to do well.
But cheap CGI looks better than cheap practical effects. Or it can be made cheaper. Maybe both.
Anyway, even Empire Strikes Back involved using computers for some work. Yep, late 70s’ computers.
It’s not one or another with these.
I think the reason for the drop in quality is moviemaking becoming corporate. Not “owned by corporations” kind of corporate (obviously that too), but “no way to get in without acquaintances or patrons inside” corporate, nepotism.