why not 和why don’t 的用法有什么不同，

why not后面直接跟动词 
why don't后面加人称代词再跟动词 
why not 后面直接加动词原型 
Why not do sth? 
why don't 后面人在加动词原型 
Why don't sb do sth? 
why not: why not give him a chance? 
why don't : why don't you choose this album?