Because if someone reports, let's say, 10.000 posts, there is quite a high chance s/he will have 300 or even 1000 of those reports will be considered as "good", but, if for reasons unknown the other 9.700/9.000 reports are pure BS, said person would still have the "good reporter" badge when only a 3/10% of those reports are accuarate. Similar to the trust system in a way
If theymos is planning to implement a badge system for good reports who takes their time to make the forum better then he will only take into account the good reports as a benchmark and not the amount of reported posts, theymos will only implement things that are logical and hence do not worry about that

the main reason theymos did not make any major changes to that report system is simply because the people who report will do so even if there is no status quo

.