Because the screen is so small, you barely see a difference between 30 and 60 fps already, even though the difference between 60 and 120 is double that, you will even notice much less of a difference if at all on such a tiny screen.
And then you get a lot of drawbacks, more cpu,gpu,battery,memory etc needed to support this as well, and since mobile powered devices have to be very careful with the battery charge they have this would be one of the most stupid upgrades on a gaming handheld.
(Unless it’s for 3D purposes)