There are too many unsourced uploads. There may be legitimate reasons for leaving the source blank, but for most uploads, those reasons don't apply. Uploaders are either being lazy or they don't understand the importance of proper sourcing. I have a couple of ideas to improve this. Do either of them sound feasible?
1) Require new members to source their first few uploads.
Anecdotally, it seems like most unsourced uploads come from members who haven't uploaded much (fewer than ~10 posts). So, for those members, don't let any upload succeed until they've put something valid in the "source" field. This restriction would be lifted after a few of their posts have been approved and they've shown that they can properly source.
One potential problem could be people typing garbage into the "source" field just so the upload succeeds. In practice, I hope no well-meaning person would actually do that.
2) Require deliberate action to leave a post unsourced.
Just like how uploaders now need to explicitly choose one of safe/questionable/explicit – the rating won't default to questionable if you don't pick anything.
Leaving the source blank would be an error that could be overridden by checking a box that says something like, "Yes, I have done my due diligence to find the source of this, and I really do not know where it comes from." This might prod people out of their laziness.
Updated by Mantikor