Because you'll try to put something up and you get flooded with spam/hacking attempts and whatnot
Because registering a domain, deploying wp, etc if not so trivial
Gmail and other "big email providers" are needed since there's no litigation against email abusers, and there's a constant flood of crap to the spam folder
Walled gardens are surely problematic but they're less trouble than going independent.
Because you'll try to put something up and you get flooded with spam/hacking attempts and whatnot
Because registering a domain, deploying wp, etc if not so trivial
Gmail and other "big email providers" are needed since there's no litigation against email abusers, and there's a constant flood of crap to the spam folder
Walled gardens are surely problematic but they're less trouble than going independent.