Because a specie is not a culture. Just looking at humans in the real world in the past and present and you'll see tons of difference in upbringing. The idea that 'Elf' or 'Dwarves' are all that same, or that species like Orcs, Drow, Goblins, etc, are born with naturally 'evil' traits is filled with all sorts of weird real world connotations, unintentional or not.
Divorcing what is genetic from what is learned would help add variety, but also avoid unfortunate implications.
The idea, at least the one I would suggest, would be to allow the DM to create their own set of cultures (which would themselves be built by smushing one of each Environmental and Societal traits together), and so the PC would just pick from the available species and one of the DM's pre-set cultures, creating a replacement 'race' block that could cover the stuff the Race pick normally does.
To a point yes, but what I'm advocating is just 1 more segment of what is already there. Instead of Race-Background-Class, you go Species-Culture-Background-Class. That way, an Elf Noble Wizard from the Norther Metropolis and an Elf Noble Wizard from the Forest Conclave won't feel like the same character, even if they pick all the same Class element.