They can, and they do. They're typically considered safety devices since they can be damaged by having excess load dumped on them, and they either are dumb, in which case they don't act like an actual load the generator is expecting and can maybe cause damage to the generator, or they're smart and can mimic the type of load the grid would actually give, but now they're expensive and need maintenance and testing in excess of what the dumb one needs.
It's something you would need for off grid solar as well, with batteries that can only take so much charge, but at the power grid level it's a much bigger task because you're in the realm of "metal explodes" power, and exploding metal is bad.