I am unsure what OBS is but it sounds like you're recording your screen via webcam and then streaming that ... My guess is the compression on talking heads is aggressive and has a different profile to a screenshare.
And even with the talking heads compression: why can't I disable it (I and most colleagues have solid broadband)? We have a nice FHD (Sony A6400 to HDMI-capture) webcam as well and the picture there is also far from superb.
But well, this all might make sense if the target is just to make sure, we don't increase their AMZ-bill too much after our uni had setup their license-agreement with them...
We were really trying to use different solutions, because of all of the problems Zoom has. We just had constant quality issues that made it really hard to hear people.
Then we tried Zoom one day and it was incredible how good it was in comparison.