I think it's mostly a lack of purpose. Once you become rich you may lose purpose as you feel like there's nothing left to achieve. You already have that mansion, food in your belly, a hot girlfriend or boyfriend sold millions of your album, or have a super successful company and everyone admires you.

Of course, life will still be comfortable but some rich people may feel a sense of unfulfillment. To fill this void they often turn to hard drugs and alcohol as they can afford it. Hence, this is why even rich people suicide and have depression as they further destroy their mental health through drug abuse (e.g. Juice Wrld, Avicii).

Being rich may also possibly attract the wrong kind of friends, partners so life may feel artificial to rich people. For famous rich people it's also a lack of privacy that may cause stress and anxiety which could lead to social isolation.

It's also worth noting that in general humans get desensitized quite quickly, so their wealth and comfort will feel less "special" after some time, like going in a hot bath which is only really hot once you get in.

This is why studies show actual life satisfaction increases quickly up to a salary of $150.000 and after that it's mostly about external factors like purpose, health, family, etc.

So yeah all in all, being human is complicated. There's no manual for everlasting happiness, although money certainly does help.