Because it’s not a habit. It’s a deliberate choice to pick up their instrument and play. Every time.
Muscle memory helps, because that seems to be separate from forming habits. Once you’ve got it, it seems to stick.
I was in a band in my late teens, and I had to force myself to practice if I was alone. Practicing with the band was much easier because it was something that I really enjoyed, but even then, I’d almost always be late, or have to be reminded to go.
This was something that I loved to do, but still really struggled with. As soon as something else came along that was a bigger distraction, the band was done.