Well, for many other producers, it's both were songs start and end. They might send them elsewhere for mastering, but not for production work.
Anything without analogue instruments doesn't need much to be found elsewhere (I'd argue also anything with analogue instruments doesn't either, but many prefer to track vocals, drums, guitars, etc. on Pro Tools or similar)
Mostly, though depends on the genre, and the kind of artist. Artists working mostly with analogue instruments, or that have huge sales and production teams, will tend to use other apps, mostly Pro Tools, and use Live as a starting point/sketchpad.
Producers and composers of pop and electronic music, that are not on the paycheck level of, say, top-charting artists, even if they still have tons of fans and a good career, do tend to do everything "in the box". It's perfectly capable. Heck, even FL Studio has created tons of "in the box" global hits...
Not so wild, people use different tools for different needs :)
I do my sketching wherever, actual production on bunch of hardware and only use Ableton to record final main output of hardware and do my mastering there, nothing else.
Producers be producing in tons of different ways :)
