As a rule, proprietary code isn't distributed widely, so there are few opportunities to include it, and as a rule, the harsher restrictions on distributing it make people less likely to not notice that they're not supposed to distribute it. It's much more likely that incompatibly licensed GPL code would be widely distributed and that it would be included by mistake.

Sure, you described a scenario where this can happen to proprietary code. It's not impossible, just less likely.