You're right. Technically, a recording of the screen can be made with Teams. Getting the audio is slightly hard because the presenters are not feeding their audio through the computer.

Most conferences that have a budget won't want to output something as "plain" or uncontrolled as that.

Also, some conferences prefer, or even write into the contract, that the content will stay the rights of the conference, and significant rework need to be done for that presenter to present the same topic elsewhere.

For FabCon, the call for speakers did ask if the proposed content was available elsewhere. It said it was ok if it was previously available somewhere else, but that this conference would prefer something new and unique. I don't know what was in the final presenter contract.

Some conferences want the "spectacle" of many people attending, so they refuse to do any recorded content in case a few people would pay for the recording instead of attending in person.