raptorattacks

Multiple answers to that question:

• There are women on Grindr - in my experience, mostly trans women or m/f couples looking for a third. So men can find straight sex on Grindr, but it's rare. Women are definitely a scant minority of the users.
• There are plenty of "straight" men on Grindr. You'd have to ask them why - I usually block them.
• The reason why dating apps marketed towards straight people don't allow this kind of messaging (and the reason there aren't more women on Grindr) is because women, generally speaking, won't put up with the same kind of shit that gay men will. A straight dating app where men could send horny messages and dick pics to every woman they saw wouldn't have any women using it.

Grindr doesn't really have a "match" system. You can send a message to anyone whose profile you can see (unless/until they block you). Think of it less like a traditional dating app and more like an interactive billboard of horny people in your area.