Most of the gaming/media/comic conventions I have been to have involved game/media/comic artists selling their own work, not people selling fan art. In fact, the presence of the original artists selling copies of their original works is generally a big draw for the convention. Maybe we go to different conventions or something (I have never been to an anime convention - so maybe that's what you're referring to). The little third-party art I have seen at these conventions is sold with the explicit permission of the original artist/IP holder. So no, it is not "obvious" to me, as someone who has actually gone to a few gaming/comic conventions before, that fan art is a huge industry or that it undercuts demand for the original art.

The fan art I have seen is generally drawn by (professional/high-end amateur) artists for free on deviantart because they like the characters or want to practice their skills.

Also, nobody is currently suing (or particularly upset) over people making art on their home PCs. People are suing over companies selling AI art generators for $billions that directly compete with the artists and stock photo libraries that were used to train these art generators.