why do all erp solutions i know have a poor design?

one of you guys surely works for a company which sells erp solutions. as i am a user AND a programmer.

i just have to ask: do you have the feeling that your UI is bad?
and if - why is it like this?

i dont want to attack someone. just want to know the reason why all of the solutions i saw have bad UI or are just "user-unfriendly" (like you would say in german :D)