If switching citizenship was on the level of ease like switching rental apartments, complex but doable, then you'd have a point, but you generally don't have that much choice in your citizenship. It is one of the last bastions of socially accepted generational discrimination. Countries try to keep you in their club and try to keep others out of it, even if discriminating between people based on where they were born is objectively morally wrong.
Not that I think it's a good idea to get rid of borders today, just that a natural extrapolation of western morality, which favors choice over birth, leads you down a path to a borderless world.