Because I'm doing some research.
Well, that's totally unhelpful as an anwer goes, thanks for not helping us help you.
Guessing from your intro, though, I suggest you google up the "Slushkiller" post on Making Light, and either read down or skip down to the part where the various reasons why manuscripts get rejected are enumerated. Go do it now.
...
Done?
Okay, so here's (a) thing about self-published books: that filtering process doesn't exist. So really genuinely good novels are mixed in with a significantly larger pool of mostly significantly worse stuff, stuff that fails at each and every step along the list you just read. People can and do succeed at SP, and it's a perfectly valid path to take, but that's what you're competing against, trying getting noticed in the crowd.
Now, having said all that, SFF is a room for discussing writing SFF and relevant, related topics, not for hashing out the pros and cons of self-pub versus trade. We have
a whole room for that, and it has stickies that are full of all sorts of useful information, which are well worth your time to read if you're considering the SP path.
Since you've elected not to provide any reason why your original question is relevant to the writing of SFF, I am going to now close this thread. If you'd like to be more forthcoming in a way that might change my mind, please feel free to PM me. Thanks.