You must have some shitty ones. Proper fitting boxerbriefs prevent chafing and your balls sticking to your thighs because they won't get in direct contact.
I got a bunch of different kinds of boxer briefs a few years ago, and I learned from that experience that there is a WIDE variety of quality and fit in this category. They really shouldn't chafe or let your balls stick to your thigh, if they do I'd highly recommend trying a different brand or fit or whatever.