Why dont providers open all ports in a CGNAT?

Forgive my ignorance if this is a stupid question, i do not study CS, just a curious person. But if every client has a NAT at their home, protecting them from unsolicited data, why not open every port in the CGNAT, this gives the option of the user to open any port they want, getting the benefits of a unique public ip without risking other consumers?

Perhaps i’m misunderstanding something.

Thanks!