It amuses me that the whole reason 2.4GHz exists as an ISM band, is that the band was too noisy to make use of.
Basically microwave ovens came before the ISM band. 2.4GHz is the sweet spot for heating water with microwaves - there's a few frequencies that work, but too high is expensive and difficult to shield, too low requires more power to get the same results.
So this band was effectively "written off" by the ITU/FCC as being too noisy to commercialise. The result is a band where you can do almost anything you want, as long as you emit less power than a microwave oven does. This makes it ideal for local-range applications that don't want to deal with the licensing requirements of 'real' bands - as long as they don't mind sharing it with microwaves. It's the typical story of a lightly-regulated free-for-all.
2.4GHz isn't a mess because everything uses it - everything uses it because it's a mess.
I didn't mean to imply it's a magic number - it's a sweet spot as a cross between power/cost/efficiency. The band wasn't authorized for communications until it was already pretty much spoiled.
It happens the other way too. I once wasted few hours debugging why 4K mode wasn't working on some Mini-PC computer. Turned out that the cause was plugged mouse transceiver into USB port adjacent to HDMI...
2.4ghz is like the duct tape of the electromagnetic spectrum...everything runs there.