You certainly /can/ self host. I've been self hosting my own email since circa 1999 on my home internet link. I've been through four different ISP's in the ensuing 22 years, but email still flows.
What often happens is that virtual hosting firms (Linode, Digital Ocean, etc.) are often used by spammer's for their hosting too, and so if you try to host by renting a "cloud vm" or "cloud server" and are unlucky to have an IP address a spammer previously poisoned, or just happen to be in the same netblock as a prior spammer, you find your new IP often 'blocked' from the big services, for no good reason than you happen to be from a "bad neighborhood". And this is usually the genesis for all the scary stories about "can't self host".
But reality is, you can self host, but you do have to set things up with all the modern requirements (SPIF, DKIM, etc.) as well.
What often happens is that virtual hosting firms (Linode, Digital Ocean, etc.) are often used by spammer's for their hosting too, and so if you try to host by renting a "cloud vm" or "cloud server" and are unlucky to have an IP address a spammer previously poisoned, or just happen to be in the same netblock as a prior spammer, you find your new IP often 'blocked' from the big services, for no good reason than you happen to be from a "bad neighborhood". And this is usually the genesis for all the scary stories about "can't self host".
But reality is, you can self host, but you do have to set things up with all the modern requirements (SPIF, DKIM, etc.) as well.