Is the premium you'd pay equal to or greater than the sum of the 15 services you'd subscribe to? If not,then you've answered your subsequent question (why haven't they done this) -- between one and all of the 15 would take a hit.
Sure, it may be overall better for each to do this, and obtain your patronage, but it's a trivial assumption to make that they've collectively run the numbers and determined it's not that compelling a path.
What's the problem, by the way, of having 15 subscriptions? Is it the 15+ emails a year to renew, the hundreds of spam emails you're likely to get (you'd still get those with an aggregated subscription model, I suspect), or some other factor(s)?