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For context, LDAC is one of the few wireless audio codecs stamped Hi-Res by the Japan Audio Society and its encoder is open source since Android 8, so you can see just how long Windows is sleeping on this. I'm excited about the incoming next gen called LC3plus, my next pair is definitely gonna have that.
It's because all the good codecs (aptX, LDAC, even SBC) all operate over the A2DP Bluetooth profile, meaning your computer's Bluetooth adapter is running in a particlar mode called A2DP. Unfortunately, A2DP is unidirectional, (ie, not Duplex) it doesn't support sending audio to headphones and receiving audio from the mic at the same time, due to bandwidth limitations. So when you open Zoom or Teams or something that needs to access your mic, your Bluetooth adapter switches to a different mode like HSP (Handset Profile) and HFP (Hands Free Profile). These profiles do support duplex connection to the mic and headphones, but don't have fancy high definition codecs. They are designed to be very low bitrate. That's why you encounter such a big audio quality difference when it switches.
Note, having said that, pipewire on Linux does support some fancy duplex modes that operate outside the constraints of what I explained above. Eg, it gives me the option to enable AptX-LL with duplex, that passes the mic through using whatever little leftover bandwith is on the connection. This doesn't work with heavier codecs like AptX-HD. I think Android does something similar, that is why you don't notice the profile swapping behaviour as badly on Android.
A2DP itself does support bidi, faststream is a A2DP BIDI profile (it's SBC, it's bad because of really bad bitrate constraints for some reason though) AptX-HD actually support aptx-ll but I find most devices just bug out when you try it. opus also supports bidi on pw.
bandwidth limtations are not an issue with bluetooth itself, you can reliably maintain 300-600kbps, which is more then enough for good quality bidirectional audio. even when using asynchronous modes, you still usually have enough bitrate for audio that would be considered fine.
im not sure why it's not more popular in modern devices, IIRC all the patents on the subject have ended, could be a limitation of low end radios