Same reason people will host and run services around IPFS, BitTorrent, Tor, Matrix, Mastodon, game rooms, and other servers even though they don't get a financial reward, and even though it may even cost them money or their time. They might just want to be part of the network, supporting it in their own way.

The protocol rewards are there to create a stronger incentive, so that it isn't just hobbyists and geeks paying out of pocket each month to secure a multi-billion dollar network.