Why do 0% APR cards & balance transfer offers really exist?

Currently I have around $6k CC debt that I've been moving between banks for the last 5 years.

Initially I had the balance on Capital One. Then I received an offer of 18-month 0% apr from Citi, transferred all there. (for a ~$160 fee) About two months before the expiration of Citi's intro APR period, I received the same offer from Discover, and for about the same fee, I moved it all to Discover... and then to Chase, now to Wells Fargo... You get the idea.

I always had the money to pay them off but I have literally no reason to do so

In total, I paid about ~$700 in balance transfer fees in the last 5 years, but made 5 times back just by keeping the same amount of money ($5-6k) in index funds & staking stablecoins.

Sure, it hurt my credit util% at first but now that I have a bunch of cards it actually started to have a positive impact on my score as well.

So I guess the question is... why do these cards and balance transfer offers exist and what's real gameplan for banks offering these deals? I can't really wrap my head around how these can be beneficial for banks in the long run.