Microwave ovens work at 2.45Ghz, a harmonic of the resonance of water (2.4Mhz), which is how food gets hot. 5.8Ghz CAN also work, but power generation enough to “cook” is very very expensive. However, you can focus that power into a beam (directional antenna) and yes, i’ve had RF burns from 600mW of RF power going into a 30dBi high gain, high focus antenna (EIRP of around 600w). Now, most people do NOT have 30dBi antennas laying around (at 5.8Ghz that’d be about 3 foot around) and they sure don’t have that strapped to their heads, nor does the typical cell phone have 600mW of power in it’s wifi radio. It’s more like 30-70mW with a 2dBi “patch” antenna in the phone. It’d take a LOT of phones, with a massive combining system (forget losses) to even come close to the power used in a typical microwave oven. Your idea is very wrong in that it’s an additive or cumulative math… far from it. Free space and non-focused power does nothing, even in quantity of thousands. Add some humidity into the mix and it really goes nowhere.

What it DOES do is “jam” or interfere with each other causing all the issues.