Because it's dead simple, and there are 50+ years of ancestry and class options in addition to tropes across dozens of genres.
Yeah, it's...not hard to build a ton of subclasses and ancestries because both of those things are relatively thin in 5e terms. Ancestries especially.
This is one of the reasons why I would prefer to have a unified system of character stuff: you have your Heroic Origin, which leads to your Paragon Path, and concludes with your Epic Destiny. Heroic Origin would combine 4e and 5e Backgrounds and 4e Themes, plus Ancestry and Culture, to define where a particular character came from,
why they adventure, what they value. Novice levels could make this something you construct as you go, rather than arriving all in one chunk*: Novice characters haven't
had their Heroic Origin yet, while 1st-level characters have done that. Novice characters are still "green"; 1st-level chars have their
start under their belt, they're no longer tenderfoots, they're at least a little experienced.
There would be several obvious pre-packaged Origins, but players who
want to customize would be encouraged to do so. Maybe you're an Elf (ancestry) raised by Dwarves (culture) on a pirate ship (background) who worked as the ship "windfinder" (theme)--a highly unusual combination but one that provides a ton of richness. Then you steal 13A's idea of a mix of class-based and ancestry-based stat boosts to seal the deal of "your education matters, but your physiology still shapes you."
*This is part of why novice levels are so incredibly useful. They manage to serve two different masters: OSR-type fans who want the distilled zero-to-hero experience,
and brand-new players who benefit from a steady, measured pace of option growth while having enough of a cushion to make (and learn from) mistakes without
having to lose a character in the doing.