Waste heat from power plants isn’t always useful because it is low grade heat… but it is still much, much, much better than the 35-60C water you could get from a data center.
Do the math on pumping 40C water any distance in a loop. It is hard to make it worth it. The water cools off and the pumping takes significant energy, plus the pump and all the piping cost something.
Then using a heat pump on it makes it even worse.
Low grade heat source is not a term I made up- it refers to heat sources under 100C.
Even much hotter low grade heat usually goes unused. You usually need a perfect confluence of a warmer source, very close need for building heat, and a willingness to pay more for environmental friendliness for it all to work out.
The COP for a heat pump doing such a transformation can be very high, i.e. for a relatively low power input you get a lot of gain.