I don't think you understand what burnout is. Your brain is a muscle, and just like the muscles in your arms and legs, it needs to be worked out, and it needs it's rest periods. If you overwork your brain too much too often it will become strained and need a break to recover. This is called burnout.

The gifted kid who isn't giving themselves the necessary breaks will study 7 days a week and keep working their brains super hard. Their brains are freaking ripped from all of the constant exercise, but they're also being pushed to their limits and beyond. Their brain is screaming for a break, it's completely beyond it's limit, and then it becomes strained and won't operate at anywhere near the strength it was operating at before. Now they're forced to rest whether they want to or not. Except society doesn't think of brains as muscles, so we don't give the really brain-strong kids breaks or make sure they get their rest days so they strain and injury themselves as a result. The injury isnt like with arm or leg muscles which tear or may cause a fall, instead it's more psychological (with of course physiological backing)

So gifted kid burnout isn't just "boo hoo life is hard" it's "holy shit I can't do a quarter of what I once did in my sleep!" except we live in a society that highly values brain work and compensates it accordingly, so these gifted kids who've burnt out are now also struggling financially and they know it!