Because you literally connect to a git repo and your site is deployed, and scales with load. Compared to managing a VM, system and application packages, config, backups and then how do you scale that?
I have what is basically a demo running on Vercel free tier, there's no way I'm maintaining a VM for that lol.
If I had a serious site, same. If I had a team, then the equation would be different.
It's like if Dropbox was an rsync server (no app) and it cost $10,000 a month for 1TB of space. Think it would still take off?
https://www.explainxkcd.com/wiki/index.php/2501:_Average_Fam...