The nice thing about frequency based regulation is it's an inherent property of the system, so as long as you're connected to the grid you've got the info you need to decide when to turn on or off.
If you're monitoring real time power consumption you then need a whole extra infrastructure to communicate this info back and forth. Of course you then have to consider how you're going to keep that extra infra online in the event of power issues.
Frequency based regulation is only telling you that something is wrong, but not what or how to fix it.
If you find yourself in the middle of a black swan event, and 15 GW have tripped offline, you have milliseconds to dump pretty much exactly 15 GW of load, otherwise more generating capacity is going to trip offline very quickly.
If you only dump 14 GW because you used historical data (which happens to be imprecise, because today's cloud cover reduced rooftop solar output), you're still going to be in trouble. A detector scheme with sensors at every substation would allow you to do just that.
If you're monitoring real time power consumption you then need a whole extra infrastructure to communicate this info back and forth. Of course you then have to consider how you're going to keep that extra infra online in the event of power issues.