"Because the transformers which convert distributed power (typically lower frequency and higher voltage) to the household power (60 Hz / 240 VAC split-phase) are made to work efficiently, one-way. Going the other way, they are considerably less efficient."
Everything on a power grid operates at exactly the same frequency. DC interconnects and other exotic technology aside, it is one gigantic synchronous machine. If load increases faster than supply the frequency of the entire gird slows down a tiny amount, as supply increase and load drops, the frequency goes up a tiny amount. Supply is continuously adjusted to keep the frequency stable. It is like a train. Locomotives tend to speed it up, wagons tend to slow it down, but it is all going at the same speed.
The POWER required has to match, input to output. On that count, I will agree with you. In that regard, it's synchronous. And if I want to grid-tie an inverter, the inverter must be able to match voltage, frequency and phase to what is being supplied to the house. And yes, when you change the load applied to an AC generator, the frequency will vary slightly, but they have feedback mechanisms on those to keep frequency within a very narrow range. But the electricity reaching my home is coming from multiple power plants; if this one varied frequency significantly, it would throw it out-of-sync with that one, hence the feedback and regulation of same.
May I direct your attention to this article? I know for a fact that the rural electrical coop, to which I'm attached, is sending out well over 1 kV to the transformer outside my house. The transformer is converting it to 240VAC 60 Hz split-phase, and it's compensating for the fact that the 2, 120 VAC channels in my home are not perfectly balanced. So the transformer is fairly sophisticated, in that regard. But sending 240 VAC split-phase long distance is going to require some VERY thick lines to avoid resistance losses. They avoid this by transmitting very high voltages (some places, over 1 megaVolt) and keeping the current down. That minimizes transmission losses. The distances involved STILL get some significant losses, but they'd be FAR worse if they were transmitting 240 VAC, long-distance.