They use it in the city heating system, which is obviously the optimal way of reusing the heat but most data centers aren't put next to such a heating system.

I'm talking about making electricity back from the heat (using a low-temp thermodynamic cycle). It has a low yield (due to the low input temperature) but it's usually economically viable when using heat that would end up in the heavens anyway.

the heat is way too low grade. typical water cooling setups only have ~50C water. that's barely anything