... and here's why

1)Cheap. So cheap that you can literally speedrun it in about ten minutes of gameplay. (If you get lucky, but gold, feathers and essence of air is easy to find/craft)
2)Who needs jump effect?
That belt gives hover about over four blocks.
3)Sprint consumes hunger.
Gliding with levitation belt doesn't and looks much faster.
4)Those pesky undead, explodings or something peasants can't reach someone so high.
Well... even wild boss can't simply hit you without unnatural abilities too.
5)Who needs elytra anyway?
Early game flight.
Because glyph of slow fall is tire 2
And belt gives unlimited slow fall.
Than try leap, cause it tire 1 flight... uh, this one works even without levitation belt, yeah...
6)Water walking?
Pff, gliding over lava.
Now nether is such a garden to walk around... in air over lava lake.

And I'm not complaining, just "joking out" everything so good about it.
Even more, it looks like improvisation with that belt because there's no word about how much useful this thing is.

Still, maybe it needs something to balance it...

Mana cost?
More expensive recipe?
Should work.

Maybe more vanilla style?
Something like...
There's limited time of use, wich could show in durability, then after timer expired belt goes in cooldown for a while to work again later.

Or should it be more complicated?
Idea sounds like: Now levitation activates like elytra by pressing jump in air. Player model become "sitting on air" and starting levitating as usual, but glide speed much slower; walking is faster then gliding with levitation.
Moreover, taking damage breaks levitation. Now it much more risky to fly over lava lake +skeletons could easily take such mage down for other mobs to finish that job.

I hope you'll find here something useful.

... almost forgot
7)Fall damage immunity by reaction.