I pop in and play this game quite rarely, so I can't say I'm an expert. But a lot of the things you guys are asking for is not as easy or "lazy" as you're making it out to be. This isn't an ALIEN game. It's a game with an ALIEN themed mode. It's built to be its own game first, so a lot of the tiny little things being asked for would be so much extra work or re-coding that it isn't necessary.
Plus, as some mentioned, visual consistency through different levels/modes is important. If I haven't played in a long time and pop onto a new map and everything I need to use looks different, because it now has to look like it's from the ALIEN universe or something, I'm gonna get confused and probably not play due to it getting too visually complicated. If this was a dedicated ALIEN game, yes, absolutely make things look like ALIEN.
A few of you are literally asking for entire game mechanics to be reworked, when you think it's just replacing a visual asset. Xenos crawling on the walls for example, is like nothing any other character would do, which would mean they have to build something brand new from the ground up. Takes time, testing and money, which this DLC is probably not funding a ton of.
The fact that we even got a Xeno in DBD is awesome. But it feels like a lot of you are complaining that a Xenomorph can dance and use guns in Fortnite... because that's the game and how it's played.