Self publishing is generally considered an ego trip.
If you think you can publish, promote, and "sell hundreds of
thousands of copies", on your own, go right ahead and try it,
but if no publisher is willing to invest, it's unlikely.
(They do know their business.)
Without a publisher behind it, even a great book is unlikely to be
widely read. (Promotion is necessary to even make it known.)
If you submit to a publisher without an agent you might be read,
and they might want to buy it. They will offer a contract.
Without an agent, you'd better have a good lawyer.
