You absolutely can choose a deity though, it just isn't required mechanically. That is a very big difference. The assumption in D&D character creation is that every character will choose a patron deity, I believe. The same still goes for paladins, so naturally that deity will be who you made your oath to. My guess is that the lack of requirement to choose a deity is part of the effort to make D&D a "generic" ruleset. If you want to create your own setting, which is low key what they seem to want by default, then maybe you don't want paladins to work the same. Plus I think having paladins differ by oath helps make them distinct flavor-wise from clerics.
On another note something occurred to me which has me worried. Minthara is apparently a vengeance paladin, or at least she used a vengeance paladin ability. I really hope Larian don't just make that oath the "evil" oath.
