AniDB

Anyone know why most harem animes follow the same format.

1 Always one girl who is a tease (usually the oldest)
2 always one girl who is either an inventor or potion maker (usually the youngest)
3 someone always catches a cold and has to be taken care of.
4 one girl who is weird (nerds, book worms, manga fetish Etc...)
5 one girl who was the child hood friend

You forgot:

6) The obvious love target who easily misunderstands situations, causing her to become violent.

It's a proven concept in the target audience! Why change that which works well. It's much more dangerous to try and be innovative and there's probably quite a few people you have to convince in order to get a go on such a project.
(Honestly, harem anime is hardly the only anime genre which suffers from this.)

sometimes:

7) The cousin

Which worries me the most.

That worries you the most?

7b) The sexy step-sister
7c) The MILF

>_<

then we gotta add

7d) the sister who turns out through some situation to not actually be his blood sister. (I.E. Kanako from "Love Hina Again")