Because it sounds cool, you silly.
splitting off of this thought - this is another reason why Bethesda is so weird. on top of their approach toward game development and their willingness to try anything as a publisher, they're like the only game-related anything left with such a simple name.
Bioware, Rockstar, 2K, Irrational, DICE, Activision, Electronic Arts, whatever - they're all flashy words, designed years ago to look cool and stand out. Bethesda's just Bethesda Softworks - software from Bethesda, Maryland. it's the kind of name that you'd see on database software in the '80s (which i'm pretty sure they did, actually), not AAA video games.
a delightfully outdated name for a mostly outdated view on how game development and publishing works, in all the best ways.