We get quite a bit of feedback from lower ranks who are not happy getting thoise simple maps "cuz it gets boring".
Also what you're suggesting is to literally take the 4v4 pool and slap it into the 3v3 one meaning the pools would basically be equal.
The whole concept of 3v3 was to be MapGen due to the ongoing discussion about ppl who only want mapgen vs. the ppl who dislike it. See all the 1v1 ladder pool discussions for that or even my earlier forum posts. Having is this way let people play either the custom maps in 4v4 or the mapgen 3v3, taking one away isn't good.
And if you're wondering "why don't you swap the pools then", that's because mapgen 3v3 is simply better than 4v4. If you see enough maps you'll notice the difference, but I'm sure it was disgussed ealier either here or on the discord so I'll leave it with this.