Because potential buyers would instantly know that they are buying stolen goods. People who would be fine with that might just as easily steal a bike for themselves, making the definitely-stolen-bike market a really weak one.

>People who would be fine with that might just as easily steal a bike for themselves

This isn't true. I know several people that don't care much about buying things with a shady history but would never steal anything themselves. The logic was something like, "Would you still accept a $20 bill if it had been stolen at some point since it was printed?"

That wouldn't hold up in court :)

If they could prove you knew or suspected it might have been stolen, you can get charged (depending on your state's laws).

I, for one, would not buy a bike with that filed off, because of the implication (and the fact that buying goods one could be expected to know is stolen is illegal). So that's that

It's inferior compared to welding a new bead over the serial, grinding it flat and engraving a new serial?

I assume it's hard to sell it as your own if the buyer isn't shady/cares about buying stolen goods?