Why is there so much MLP fan art that is this technically proficient? Like I'm familiar with lewds (they're trending on this site all the goddamn time) but for some reason I've only seen this level of professionalism on MLP fanart

Because a shit ton of MLP fans are artists (whether they make art like this, music, writing, or videos) and with an audience as large as the MLP fanbase, you have a lot of artists with a lot of incentive to show off their skills and able to see what they're peers are working on as well to build off of. It's kind of like an art club, really.