You know the answer to that already NY44m! Conventions are a $$$$$$$ maker! Plus the gathering motivates the people just by riling them up.
Not just the money - conventions are great for applying social pressure to get people to accept some of the more extreme indoctrination. A lot of these videos are things that an average, unthinking JW would object to were they to view it privately. They then might seek out someone else that they know who is of like mind to discuss their objections. Having JWs all view these together prevents this - they're sitting there with tons of other JWs around, all nodding along (or, at least, failing to make a fuss about it) so they do the same. They see other people listening (even their friend that they might otherwise assume would have similar objections to theirs) and outwardly going along with it so they have to silence their concerns. The fact that they then don't speak up triggers cognitive dissonance (it's wrong but I didn't do anything about it) that influences them to adjust their opinions (well I didn't do/say anything about it...so maybe it's not wrong).
Even if they managed to make up for the direct financial losses of doing without conventions, I suspect they'd end up with a greater rate of attrition of members as they move to viewing these in small groups or alone and discussing with people that might be more open in their thinking. If they're smart, they'll never get rid of conventions. At most they might go to 2-day conventions, but I suspect that's a bad move on their part (they'd lose all those great experiences about people being fired for taking friday off and then getting a higher-paying job with less hours on the same day!). The one-day assemblies made sense because many assemblies are held in their own buildings and doing one-day assemblies would probably allow them to consolidate/sell many buildings. But they wouldn't get the same benefit from reducing the length of a convention until they, too, went to a one day, and most conventions are held in rented facilities anyway. Anyway...bit of a tangent there.