Do people with tennis courts in their garden actually use them or is it just a "look I'm rich" flex??

I've played at a couple of wealthy people's tennis courts. A lot of those who have their own court(s) are not serious about tennis, but there are exceptions.

we don't even have room for a pickle ball court but wish we did.
How about table tennis?

Id honestly love that but an outdoor gym instead

I have a croquet in mine and I don't even know why.

just like swimming pools, im sure it quickly becomes a big regret, especially after the kids grow up and move out

I would check their physical shape.
If they look like they aren’t shy of any desserts;
I would go with status symbol.

They use them 🙂

They are rarely serious about tennis. The serious ones want to play against many other tennis players. That requires full facilities.

if i had a Tenis court in my yard, id never do Tennis on it, but TONS of XXX things

They get it to display their class status.