There is no such monopoly - just look at your inbox, and check the headers of the emails you see. (I just did checked a random non-spam mailing, and it was send by cmail9.com - which is pretty suspicious domain if you ask me.. and yet here is it in my inbox)

Sending emails is hard because there is no central authority. Instead, each provider checks if the sender does some “best practices”. And the exact details vary - so if you got it working once with gmail, then there is no guarantee it will work next time with live.com. So you got to do them all.

For example, I have heard an unconfirmed rumor that one of the major provider started penalizing email sender machines without IPv6 address. If I had to maintain a mail sender, this’d mean have to upgrade my architecture to use IPv6 ASAP, which is non trivial at all. This is why most people outsource email sending.