They don’t have screamers because they need something to bring more zombies to your vicinity and they just can’t think of a way to do it without a special zombie. They have screamers because they like special zombies and the idea of a screamer is cool and they purposely want that kind of a zombie in the game by design.
Look, I get that there are some of you who abhor anything other than plain normal zombies but you better get used to the idea that you’ll need to mod your game for that because the developers love special zombies and boss zombies and intend to keep them in the game. In fact I’ve seen discussion about at least three additional special/boss types that aren’t in the game yet.
You are barking up the wrong game if you are looking for non-specials. This one has little girls who scream, cops and vultures who vomit acid, spider zombies who leap super far, Ferals that run during the day, all zombies that can pound through concrete, demolishers that explode, a giant mutant pig, ___________, ____________, and _______________.
Now we just need a mutant partridge in a burnt pear tree and we’ll be complete.
In any case, if you hate special zombies mod them out because they are here to stay.