Why do people mine crypto?

Because they can.

There is no system in place that decides we don't need something. There are only competing systems creating ways to make money or people (basically money) grow more money and people.

Bitcoin mining as a percentage seems less important than its existence as something that wastes resources for the sake of wasting resources. Green or not, that's energy that either doesn't need to be used, or ends up being taken from someplace poorer that probably could do something useful with it (rebuilding Pakistan's infrastructure, powering electric farm tools, distilling fresh water for productive people).

Whether Bitcoin or Wall Street robotraders are shuffling the electrons, the electricity is all tied to the grids, and we have to stop treating the energy being dumped as though there's intended purpose to the distribution except making money and wasting time for an Invisible Hand of ignorance.