Because of the testing and science involved. It's a continual thing for many years when there's a spill or a problem. The courts are ineffective means to monitor and enforce for that reason on most issues. It takes informed personnel on the job just doing this. States could do more but states haven't demonstrated much interest in rattling the cages of their big important industries because the states need the jobs. That's why the EPA was formed. The problem isn't EPA as an agency, it's the leadership. A perfect example is the failure of the EPA to spot the problem even when informed with the drinking water in Flint. That specific disaster is the REASON we have an EPA yet they failed to handle it properly and in fact stated the water was fine when all you had to do was look at the water in a glass or a jar it to know it wasn't fine at all.

EPA can be fixed and Trump will fix it.