Because there's a texture to even the worst practical effect. You're looking at something that exists outside of a computer screen.
Texture is irrelevant, it can still look fake and dated.
The problem here is that you're inverted from your claim: "to even the worst practical effect."
Nope, you will only compare CGI against the absolute BEST practical effects.
The comparison isn't even fair because practical effects go back for over a century, the amount of work put into that craft is huge.
Comparing the best examples of THAT against any random example of early CGI is misguided and smacks of entitlement.
You realize that a technology will stall without experimentation, right?
What if all the "live-theater fans" complained constantly and shouted down movies + special effects as "fake" back in the 1920's and got it all so unpopular that it was halted?
They would have destroyed all the fake effects-heavy movies you referenced.
Oh, and the Alien effects are extremely dated. They also show their budget. Go see an amazing print of Alien on a huge movie screen like I did a few years ago.... I was shocked at how OBVIOUS the practical effects looked. Even the Nostromo going through space looked exactly like nothing more than a toy. I don't think the right lenses were used for shooting it. The chest buster skittering across the floor attached to an obvious rod is quite fake looking. The cut from Ripley setting down Ash's super-fake head, to her standing in front of Ian Holm neck-deep in a table, is a jarring cut with two different scenes.
There were actually laughs in the theater on that cut. These were Alien fans who paid money for this screening.