Okay, so this is going to be a really quick entry, but with all the recent nonsense I have been asked this over and over again by people claiming sexism. They answered this at PAX, and while I’m pretty sure you can find a video of the entire panel online, I’ll save you the time of searching through it for the answer. That also means my answer isn’t going to be as thorough or coherent as when they answered it, but it’s a general idea at least.
Basically, the answer is that while there is art direction and stuff, they don’t want to reign over the artists like dictators (for various, logical reasons). As such, the artists have a certain amount of freedom to create images as they see fit. Historically, most representations of angels have been female (Which always struck me as odd since the only biblical angels I can name are all males). Likewise, most representations of demons have been males. This is also why the majority of cards (outside of Mirage) depict only white people. It’s not because they have to, it’s because that’s what the artists are turning in, and unless there’s a specific reason why a character should have a certain skin tone(Teferi in Time Spiral, being from Mirage and all) then they don’t want to shackle their artists and impose unnecessary rules for them.
If you want a better articulate answer, as I said you can probably find video of the PAX panel. Just wanted to have this written somewhere I can point to since people keep asking me this.