The current facilities available to curators on especially Commons in order to do review of new file contributions is rather arcane. Mostly it relies on excluding what bots have contributed and making use of [[Special:NewFiles]]. There are some tickets that show that this is a problem, including the community wishlist request T120453: Copyright violation detection tool for Commons as well as T121870: Improve Special:NewFiles and T123517: Automatically check Commons uploads for possible copyright violations.
Experience (see T120867, the uploadwizard cartoon etc) seems to indicate that we cannot teach new comers the complicated rules we abide by up front, so to invite more people to upload means more work in curation, cleaning up after those new comers.
When curators need to invest a lot of time to cleanup after less experienced users, then this leads to frustration and it takes away from their ability to invest their time into projects that they would rather occupy themselves with. Mostly everyone feels frustrated, because it seems clear that automation and better software could put a considerable dent into many of these problems, yet we have not applied it.
This problem is frustrating other efforts in the stack, as has been witnessed by projects like mobile file uploads, cross wiki uploads T120867: Large amounts of unwanted files (mostly copyvios) uploaded via cross-wiki upload tool (A/B test of different upload interfaces) etc.