why is it ok for marvel to do a cgi vilian but not for dc?

Speaking as a Marvel fan:

I'm not impressed with the CGI villain I see in the ads.

Because the DCEU has had more than one terrible looking CGI character as a main villain? Villains that came out looking downright awful and were as uninteresting as it possibly gets.

Also, you obviously haven’t been paying attention but Thanos’ appearance has been described as not good by, yes, even Marvel fans. That should tell you something. But terrible dialogue (Brolin sells his lines) and “overused motivation”? (even though his motivation was set up years ago... what other motive did you expect?). Agree to disagree. The way Thanos looks isn’t the least bit problematic for me, but here’s the thing: Infinity War has yet to be released. Everyone’s already seen how pitiful Steppenwolf was, but Thanos? Uh, yeah, we haven’t seen him in all his glory yet. Marvel hears their baddest villain to date looks funny? They have time to work things out, son. And they will.

Furthermore, Thanos has been mostly computer generated since he debuted. In what other manner do you think a character like him was going to be designed? He’s always been animated and somehow he still managed to get fans excited these last few years. But, let’s say this hugely anticipated villain disappoints... you know why he might get a pass? Hmm, it’ll likely be as a result of Marvel having a bunch of critically successful and popular movies to their name. (They’ve had an undeniably damn good run for two years straight.) DC? Aside from a single movie, it’s been the absolute opposite for them since they launched their new universe.

The character being CGI in and of itself isn't an issue. Hulk is CGI and Iron Man is mostly CGI as well. The problem is when the CGI looks bad. The finished Justice League movie had a villain that still looked cheap in the eyes of many while people tend to think Thanos looks good.

As for the terrible dialogue, we haven't seen the movie, so it's hard to say if he's written poorly. The same goes for his motivation. We know Thanos wants to get the Infinity Stones, but we don't know the purpose just yet. In the comics, he wanted to wipe out half the universe as an offering to Death herself but we don't know if or how they will change it for the movie. Also keep in mind that Thanos is a popular character while Steppenwolf is very obscure, so naturally a lot of people will be excited for one and not the other.

Because some people are hypocrites. Like the people who say every Superhero movie needs to be dark and gritty cause all super hero comics are dark and gritty which is a blatant lie.

Thanos isn't completely CGI. Brolin's eyes and nose are still there (similar technique as Davy Jones from POTC). Several of the shots look completely real, particularly the one after he puts a gem into the gauntlet and then looks it over.

You're also not looking at finished product, and art design has a lot to do with it. Even if they were practical effects, Thanos has a more striking appearance than Steppenwolf.

I see your point but I think what you're missing is that, although some of Marvel's CGI is spotty in places, overall they've taken care to make it as good as possible. In the DCEU movies, the uneven, blotty and sloppy CGI was mostly due to those movies being rushed by WB to meet their premiere deadlines. Care was not given where it should have been but then that's been WB's fault with everything DC related, from poor scripts, bad dialogue and spliced-together scenes.

I liked Justice League but hated Steppenwolf. It didn't help he was a boring character but the CGI was ten, twelve years out of date. It'd have been easier to have Ciaran Hinds in a costume.