When I was a just starting out, I lived in the big city where I could see great players every night of the week. I idolized nearly every local player of that time. They were all excellent, but one in particular stood out, and he became a bit of a mentor to me. He was an established world-class musician when he arrived in town, and he was able to quickly fill his schedule with studio work. I could see him play most nights at one of the jazz clubs (which were more plentiful back then). He and many others wanted to dig into some jazz after a day filled with jingles and TV themes.
I began having lessons in a private rehearsal studio. It was pretty funky: a set of drums, a couple of cymbals, a few music stands, and 5 layers of carpet on the floor. It turned out that this studio was maintained by my then hero and friends. They would get together and play whenever possible. Why? To keep in shape, to work on new stuff, and to play 'their' music.
OK, full day of studio work, jazz lounge gig in the evening, sit-in at the after-hours club ... and it's still not enough! Well, some of us really are that obsessed with playing, but 8 or 10 or 12 hours a day? Sure, because sometimes it's the only way to get in the game and to stay in the game.
Even though my hero was highly skilled and well connected, he knew he had to get out there and play if he wanted to keep his edge, both technically and career-wise. Plus he just plain loved to play. Commercial studio gigs can be great work, but your other skills -- your jazz chops, your double pedal speed -- may suffer. So the jazz gigs and casual sessions make it possible to stay in shape.
You get better at what you do, and if you hope to be a player, then you've got to play and play and play ... and then play some more.