Gear?

Mics - are extremely important. The right mic for the right job.
Mic placement is extremely important. How many mics, what does the room sound like? What else does the mic hear? Is that OK?
Pre-amps and other electronic gear are extremely important. What are the setting you use for the pre-amps? What about compression, or limiters? How do you adjust their settings?
Mix? What about EQ for the instruments? How do you carve out the frequency range where each of the instruments "live" within the mix and can blend with one another without "muddying" the sound or detracting from another instrument?
What is the proper reverb to use or other treatment to be used and how they adjusted for each track?
How do we fix various errors - such as groove or beat errors? pitch errors? vocals errors etc???

There are so many many little factors that make a great performance become a great recording that you cannot learn them in your living, working only on your own music in a short period of time. No matter how good the gear is, how cheap it has become and how much of a "knack" you have for this sort of thing. It takes years to get good at this. Lots of them and lots of hard work - just like learning play music in the first place.

I'm a pretty good guitar player, but I wouldn't dream of taking a pro gig as a bass player next week. I know enough to know that I don't know enough to do the job well.

I've worked on a number of recordings. I could probably "produce" a small and simple project on my own - but I'm a musician. I want my recording to be as good as it can be. I want a pro working the engineering and the producer roles when I make a recording.

Mark Twain said the difference between right word and almost the right word is like the difference between lightning and lightning bug! I think the difference between what most of us can do at home and what we can do in the studio is along the lines of Mark Twain's comment.