I don't understand why it's set up like this. If I want to, for instance, just see "female deer", it makes more sense to use the one word "doe."

Because female_*species* tags for every species would create a shit ton of tags (it would also mean making male, andromorph, gynomorph, herm, maleherm, and intersex species tags, for all species). Having 'doe' or 'bitch' would give people who want to make gender_species tags a precedent to point at.

ebonyconnoisseur said:
I don't understand why it's set up like this. If I want to, for instance, just see "female deer", it makes more sense to use the one word "doe."

Nope, according to Google, doe can also mean "a female hare, rabbit, rat, ferret, or kangaroo."
Unless you want 6 different tags for doe, it would be easier to just alias it to female and search for the specific species you want.