They usually contain thousands of items from different plugins, designers, etc. Sorting through them all is impractical.
Errors are not just about protecting a developer from himself, but more importantly, protecting developers in a team from one another.
If the compiler can't produce an error, the whole feature is pointless.
That being said, I don't think warnings would be a good way to go on this case, because the sheer number of warnings generated by these new rules in existing, large (and mostly bug-free) projects would make it very difficult to reestablish a zero-warnings state. Developers on the team will just get undisciplined due the the broken windows effect.
However if your build is spitting thousands of warnings and no one cares, that sounds like an issue someone should be looking at...