I'm going to hazard a guess.

If we assume the numbers for a sort of medium-size nuclear plant, we've got a total power generation of 500MW.

Dumping that kind of power is... hard.

If I haven't missed an order of magnitude, that's enough to boil an Olympic-sized swimming pool in about 25 minutes (500MW to boil 2.5ML of water, assuming STP).

So you'd either need to build and maintain the infrastructure to burn that kind of energy when it isn't needed (not cheap) or just sell the power at a loss (likely cheaper).

Not at that levels of output. With large enough machines it's "we're going to spend X to spin everything down, do maintenance checks, spin back up" vs "we're going to pay you X/2 so you take the output and we can keep going". The X may be spent on extra workers, on extra maintenance, on replacement for posts that wear out faster, etc. It's the cost of unusual operation.

And once you're generating there's no great way to just "disconnect" in many cases. If you disconnect the load, the charge still has to go somewhere. Preferably not into nearby equipment. Even the safety switches are non-trivial when dealing with high power - if you break a cable connection, the electric arc will still keep them connected.

If I remember correctly there are rules that the estimated demand and actual generation have to match pretty closely and the overage can only be within a few percent tolerance. You're not allowed to just generate electricity that there isn't demand for.

1. Energy can neither be created nor destroyed.

In practical terms the energy has to go somewhere. Maybe they could build massive dump resistors to burn off the energy that way but doing so would only increase load on everything (shortening component lives) for no gain.

No, electricity can't be routed to nowhere, that's not how the grid works. The amount produced and the amount used have to stay in balance.

There is a bit of leeway in that turbine power stations will run a little bit slower or faster depending on exact use (resulting in a grid frequency a little bit faster or slower than 50Hz) but the difference has to be made up quickly.

(not an expert, this is badly remembered from Internet articles so probably hurts to read for real experts, I'm sorry. But I hope I got the gist sort of right.)