An important premise of nostr is interoperability, both in technical and social terms. If clients don't run relays their defaults have to be provided by a third party, reinforcing the social cobtract that if you run a relay and don't actively protect it, anyone can connect. Like how opening a connection to port 22 is legally considered hacking, but opening a connection to 443 isn't. It also distributes risk posed to the user and encourages technical ibterop as well.