I think the biggest reason is paint consistency. Even many experienced painters have problems with this. At least hobby paints will get them most of the way there.
Hobby paints also have some other advantages. There are far more color choices, which means less mixing of paints to get the colors they want. Many times they can pick out a base color, shade and highlight, which makes it even easier. At least several companies directly market their paints in that way (Reaper & Foundry come to mind.) Then there are the specialty items, like the washes. Washes can be replicated to a certain extent, but even after many years of experience, experimentation and trial & error (and internet research) I've still not been able to brew up a wash that is as good as the GW washes.
Between those options, it's not just a matter of quality of application of the paint, it's a matter of time. You can easily double or triple the time spent painting if you're constantly thinning & mixing paints.
This may be personal preference, but I don't care for the finish of most artists' paints on figures, once they're dry. This has less to do with the completed project (as you can always spray Dullcote or similar products) but has more to do with layering further paint on top of previous layers if it has a glossy or "plastic" like finish.
Those are the reasons that first come to mind.