Fewer, and lower discoverability. If you ban a sub, the mods have to bother registering a new one, and if you ban the mods too I believe they'll need to 'break in' new accounts to do that. Once they do, it'll just be an empty sub sitting there passively until they promote it on other subs (where it'll draw reports) or on other sites (which is a hassle). And without active moderation, spam users destroy spam subs by overwhelming their intentional content.

On tumblr, a new account just takes a new email, and they can be 'promoted' easily because following real users or reblogging their content with ads sends them notifications. It's pretty much the ideal structure for automation-friendly spam.