If regular microwave ovens are "off" a bit more than half the time it's running, why hasn't Wifi in 2.4GHz detect for that and try to put packets in those times? Or does the IEEE expect that most microwave ovens will go inverter and thus make such a mechanism less useful?
They do, in a sense. Wi-Fi won't broadcast if it detects that the channel is "busy". This is why if you have many radios nearby on the same channel, your performance sucks, because it essentially waits for a break in transmission to avoid collisions with other Wi-Fi devices.
Why don't they don't do this with other sources of interference? Probably because it's a little pointless. In the case of microwave ovens, they SHOULDN'T leak enough to cause significant problems with Wi-Fi (see my comments above).
The way TCP/IP is designed, if the client doesn't receive the expected response within a certain time, it just times out, disconnects the session etc... Same for your Wi-Fi access point, it won't just buffer every single TCP/IP packet that comes its way, if it can't deliver it within a timely manner, it won't deliver it at all. If your Wi-Fi router or access point had to wait for all sources of interference to cease in built-up areas, no one would be transmitting anything. Wi-Fi is a half-duplex system, while a radio is listening (or receiving data) it's not transmitting.
Which brings me to another point that I feel I should mention. Most consumer-grade Wi-Fi routers will have an "auto" channel function. Usually, this only operates at the time the router boots to listen for the best free channel. It does not continually monitor the spectrum and channel hop. For it to do this, it would need a dedicated radio that does the monitoring or disconnect all clients from the network, monitor the spectrum, then allow all clients to re-connect (the latter is obviously not ideal).
Most enterprise grade wireless gear will either have dedicated access points connected to the controller through various parts of a building/campus which act as "monitors", that's their full-time job, to provide the controller feedback on channel interference and other radios in that area so it can change the channel on nearby access points. Alternatively some dual-channel access points will have a third and sometimes fourth built-in radio to do this, like the
Ubiquiti UAP-AC-SHD.
The auto channel feature on consumer gear is next to useless (like MAC address "security"). Let's just say you had a local power failure and all your neighbours lost power too. Assume your router is the first to boot up and decides that Channel 11 (on 2.4GHz) is the most "clear" so it picks that and starts accepting clients to connect. Then all of your neighbours' routers boot up, some have auto channel (which would at this point avoid Channel 11) but some are configured to only use Channel 11 (which is very common). All of a sudden your "clearest" channel is congested and performs like crap. Your router won't detect this and change channels, it will just continue doing its best in an RF-noisy environment.