This has been my experience as well, and I think the reason for it is what you state.
But I'll break it down even more: what makes someone highly skilled in an an area is a combination of learning everything they can about the field, and spending countless thousands of hours practicing.
If you're doing a thing because you love it, you'll naturally do both without even realizing that's what you're doing -- because to you, what you're doing is play. You're having fun.
If you're learning a thing because you want to earn a living doing it, but have no special love for it, you're less likely to spend nearly as much time learning about it expansively or actually practicing, because what you're doing isn't play, it's work.
But I'll break it down even more: what makes someone highly skilled in an an area is a combination of learning everything they can about the field, and spending countless thousands of hours practicing.
If you're doing a thing because you love it, you'll naturally do both without even realizing that's what you're doing -- because to you, what you're doing is play. You're having fun.
If you're learning a thing because you want to earn a living doing it, but have no special love for it, you're less likely to spend nearly as much time learning about it expansively or actually practicing, because what you're doing isn't play, it's work.