I've often wondered?

I've often wondered why when some side actresses wish to be main actresses, and they are good and good looking and good in acting, why producers/directors will not sign them to be actual main actresses in the films? I am saying this, because I believe that we all have a certain characteristic bearing and a look and basically character that fits a certain specific role, so then why would producers/directors not bring a story line etc., to that effect and sign these side actresses as main actresses when they desire that so much, because we would have that much rich and extensive texture in movies? We'd have so much variety. For example: I think Alia Bhatt could be so good for making a movie on Anne of Green Gables. I know Alia Bhatt is main actress, but I'm just giving an example. Also, why producers/directors don't match beauty with beauty in gals and guys(actors/actresses)? Because movies will be liked so much more then.

I decided to leave the forum guys and gals. Bye!