As someone who both writes and plays, the lucrative part is nearly always in the composition and not the playing.
I have a back-catalogue of 45 years worth of songs, and although most of the songs that aren't being performed by my current band only earn a few pounds a year, it all adds up, and it only takes a couple of songs to become popular either from my band or someone deciding to do a cover and I could be set for the rest of life. I'm certainly not going to be scrabbling around for paying gigs in covers bands just so I can pay the bills, because that would kill any enthusiasm for performing music that I have.
For me, income from writing a song is like buying a premium bond. I'm know that I'm very unlikely to win a big prize, but if I don't do it then my chances of "winning" are definitely zero. But each additional song is a very slight extra chance. And if by some miracle one of my songs does become successful, it doesn't affect my current musical activities. I can still continue to do what I want right now and not have to bow to the pressures of having another "hit".