Children's interests can be fickle. Learning a skill takes a serious investment of time, and a child left totally to their own devices could end up bailing on a lot of interests prematurely, because they never get over the initial hump. This is a natural part of parenting, to teach your children about investment of effort.

The horror cases like GP's "9 years" are essentially a miscalibration, failing to recognize a suitable stopping point.

"Cheap babysitting" is not it. Lessons are short and more expensive than childcare. Enforcement of practice time is a hassle (and a balancing act!)