That is a fairly recent limit, imposed by the program.
By the DVD-specs., the max would be 99, and I think that was the old limit. Not sure why Adobe limited the Chapters to 25, but my guess would be that because of the semi-automatic nature of it's authoring, having so many possible Scene-Selection Menus could cause a problem with the Menu Assets - the DVD-specs. allow for only ~1GB for ALL Menus. If one had Motion Menus, with Audio, then it does not take THAT many Menus to go over the 1GB limit. That reasoning, however, is just speculation on my part though.
Sorry for the bad news.