You're not totally wrong; they could operate that way, wherein victims of theft would report what people have re-uploaded as their own. But the problem is, this puts the onus of policework entirely on mod operators, who have their own lives and livelihoods. Imagine you wrote a mod as a hobby for four years, spent some time abroad, came home, never heard about the mod workshop stuff, and then discovered that your mod blew up in popularity under someone else's name. Plus, Valve would need to moderate and figure out who is telling the truth (lest a bad actor make a fake claim on a legitimate mod seller). That kind of situation is often unrecoverable.

You can even read stories about the wars that photographers have over this kind of thing - photos are the kind of thing people pass around like candy, even though some amazing ones take tons of expertise and effort for the photographer to take; they often struggle to get websites and magazines to pay the ad revenue they're due for each time they're shown. It's much like the mod workshop would have been - a very "low friction" environment for reuploads.