I thought the reboots were in order to retain property rights, and that with 2-3 years between reboots aimed at teens, there are a new batch of teens. So it seems like a win-win: renew property rights and lure in the new batch of teens.
That's specifically true only for Spiderman I think. Reportedly[1] Sony loses the rights if they go too long without producing a Spiderman film.
1: This is treated as fact in many places, but was very hard for me to source. The most definitive statement I could find was contemporary with the MGM/Sony dispute being resolved:
> The Marvel-Sony contract requires the studio to begin production of a Spider-Man feature quickly; the actual deadline could not be obtained. It also places Columbia on a short leash in scheduling sequels, requiring in some cases that financing for a sequel be arranged within months of the release of the previous feature[2].
