Having zombies doesn't mean the games are similar.

Dying Light, Resident Evil, Left 4 Dead, State of Decay, Walking Dead, and Dead Rising have nothing in common other than having zombie as the main enemy. They are totally different games from each other, totally different experince. 

We have humens holding guns and acting the same in almost every game. Why is that different?

You're missing out!

Looks like Square Enix is showing a game behind closed doors to some Youtubers tonight, possibly Deux Ex: Mankind Divided. Maybe we'll get some previews from Youtubers before E3.

LMAO... just scheduled the craziest thing to do on my birthday ever. May 18th? Spending it with Square Enix.... ?!?!?!?!?!?!?

— Jesse Cox (@JesseCox) May 15, 2015

Spending my birthday evening with Square Enix....
I just found out what I get to see o.o! #excited

— Jesse Cox (@JesseCox) May 18, 2015

Don't know just how many Youtubers got invited besides him. The Deus Ex part is speculation from me because I know Jesse is a big Deus Ex fan and he said he's excited.

Zombies just aren't a compelling enemy to build so many games around. They are literally brain dead enemies, slow, predictable, not in the least bit intimidating, and they don't provide for a good story or a main antagonist so you always end up with the same boring scenarios over and over again, and despite all these weaknesses almost every single developer makes the mistake of throwing them centre stage of the act instead of focusing their attention on the human characters that could make the whole thing compelling......no it's gotta be all about zombies, brains, and the cliche military faction or private corporation that caused it all.

Zombie games are some of the weakest in the industry