Because the grid as a whole is dumb as fuck. Sure, you got measurement stations doing very fine frequency analysis, to the point you can use grid hum to timestamp audio recordings [1].
But you rarely have realtime insight at individual busbar level for each substation or heaven forbid consumers. Best chance you get is 15-minute consumption pings and voltage/frequency metering at large plants.
That means if you want to track down bad actors like the batteries here or the Kosovo/Serbia bullshit [2] that dragged down the frequency on the entire ENTSO-E grid (which is quite the challenge!), it takes months if not years.
[1] https://en.wikipedia.org/wiki/Electrical_network_frequency_a...
[2] https://www.entsoe.eu/news/2018/04/03/frequency-deviations-c...