Because they aren't built to do the same things.
ROM is typically a low-capacity storage for built-in configurations that the computer needs to load when you turn it on. Also, despite the perpetuation of the term "ROM", current ROM chips aren't actually read-only, since some things can be stored in them, but like I said, it usually has to do with basic configurations of your system.
RAM, on the other hand, is in charge of storing all the data that you are using while your computer is working. It is of a much larger capacity and its storage is in many ways responsible for your computer's performance. If you have little RAM in your computer, and you start opening lots of applications and stuff, your computer will slow down and eventually crash because RAM can't handle that much. However, if you have a lot of RAM, your computer will be able of handling more tasks at the same time.