"Then... why not let players choose to keep their kits?"

Already been debated to death on these forums so I'm not sure why it needs repeating, but the balancing issues for this would be immense.
You're balancing level 50 kit at levels 15-50, all of which need different adjustments. (a level 50 kit at level 15 might need a -25% damage modifier, while a level 50 kit at level 30 might need a -10% damage modifier, etc.)
A level 60 kit at levels 15-60.
A level 70 kit at levels 15-70.
A level 80 kit at levels 15-80.
A level 90 kit at levels 15-80.
Ensuring that at each level differential, you're outputting the same amount of dps.
None of that is easy when you have compounding effects of GCD potency increases, additional oGCD's increasing APM, and stacking 5%/10%/15% damage buffs, all at different level ranges for different jobs. So you're doing this on a per-job basis.
That's the manpower of several stat squishes at once, and they took an entire expansion cycle to pull that off, which cost us an Ultimate.

Without putting that work in, a level 90 kit would deal nearly 4 times the dps output of a level 18 in sastasha, or nearly 3 times a level 50 player, give or take depending on the job. Ensuring that all newbies are dead weight in any content they play, and ultimately fostering a culture of kicking newbies.