Email and other Internet infrastructure is more analogous to the phone company than news publishing, because you do not need to "peer" with other newspapers to get published.
Imagine if Bell was touchy about letting "undesirables" use their service.
The internet infrastructure is completely controlled via local monopolies and bureaucracies, and this extends beyond ISPs. The entire DNS and PKI system for example is simply a racketeering scheme for registrars and CAs. In general the internet (from the hardware to application-level) is structured in such a way that is weak to sybil attacks, and this requires central (usually corporate) entities to discriminate on traffic.
EMail is one such example: there is no meaningful way to filter spam, so the peering relationships between mail providers break down unless they each implement some sybil-proofing on their own side. The end product is that everyone is forced to use a provider like Google, and play a corporate-political game to stay in good favor with their peers. This is because there is no spam-resistance on Email application level
We've seen a similar situation this week with a controversial forum called KiwiFarm taken down by DDoS. They need to maintain a political relationship with their DDoS-protection provider, CloudFlare, who also controls a massive chunk of the internet infrastructure. This is all so that there can be a central entity that discriminates on the traffic, as there's no spam resistance features on IP level.
You can set up your own mail provider, but if GMail blocks messages sent from your new provider, it will never overcome network effects needed for success.
