What's hard is actually making sure your emails get delivered and don't end up in the spam folder.
Starts with the easy stuff like DMARC (needs DKIM & SPF), goes further to properly warming your IP's, then to properly parsing hundreds of different error response messages and good retry scheduling, and to complex problems like individual mail receivers spam and max-throughput rules.
Also these companies sell you the ability to easily make campaigns targeted at a specific subset of your contacts.
As for sending out mail, one of the difficulties that MailChimp and similar services take care of is keeping the servers' IP addresses out of spam lists.
It is dated it seems, but still does a job.
There are indeed issues with delivery, but you just work through these as they arise. Also, there is no monetization involved, so I do not care that much about delivery rate. But if your profits are directly tied to delivery rate - hoting own SMTP server might not work as well.