Also, orcs are ugly, and who wants to look at an ugly sprite when you could be looking at a sexy human?
The orc male portraits look absolutely amazing, I don't know what you're on about.
That said, preps probably are a big part of it, and the % damage items you can pick up from preps are typically better than the base damage items. Dwarven Gauntlets are just a better prep item than a Fine Sword, and Perseverance Badge or Bear Mace are both simply better than the Sword from the Blacksmith. Extra attack boosters are just gravy at that point.
In Purist games or dungeons with extended end-games (GT mainly) humans are probably better, but for everything else I find orcs just easier to play and more powerful over the course of the dungeon as a whole. While sometimes I find myself saving glyph conversions for later, most often I do run across as least a couple glyphs that I know I'm never going to use on a given character, so it's easy to just sacrifice them at that point. Even moreso if you're playing one of the tier 3 classes, since they tend to have the main glyphs that they need anyway.
Orc paladins are amazing, especially with prepped GG. Absolution + orc conversion bonus + humility = absurdly easy time getting significantly higher level monster kills.