Q: Why don't Thai restaurants simplify the 1-5 heat scale?

Dear Vanilla Pop:

First off I just wanna say that I love your show. You guys are great.
I've been wondering... why do Thai restaurants give you the option of 1-5 hot when you order a curry dish? Why don't they just offer "mild, medium or hot" like the green Chile you buy here in New Mexico, and make it a whole lot easier for us to order?

It's so confusing with so many choices.
Thanks in advance. 
Sincerely,

Keep It Simple Thai restaurants