"why not do sth""why not to do sth"which is right

why not 表示"为什么不",是个固定用法后接动词原型.

why not do sth!!! is right

应该是why not do sth=why dont't you do sth

why not do sth=why don't you do sth=you'd better do sth
它们都表是建议的用法。

why not do sth 是指为什么作那件事，其中那件事已经发生了，
而why not to do sth 是指你为什么不去作那件事，其中那件事还没有发生，等你去做呢

why not do sth!!! is right