It's the next best thing to impossible to run your own email server these days.
Having worked from the mail sending ISP side of things, this is definitely a 'the bigger you are, the more likely the giants are going to work with you' sort of situation, but Google sucks so much harder than Yahoo (pre MS acquisition) and Microsoft ever could on this front.
When you have 20+ shared web servers and 1000+ customers, you're going to have spammers get out into the wild sometimes. People leave vulnerable code on websites, people have easy to guess SMTP passwords, sometimes there were security gaps in systems we managed that got exploited. Yahoo had an easy to complete form to fill out to detail what you found, how you fixed it, and ask to be delisted from their spam filters. Microsoft has a robust set of mail admin tools that let you see all the spam their filters detected from your IP space (as proved out by WHOIS records) so finding which servers or accounts had been compromised was pretty easy.
Google gives you just about nothing. They're too big to care. You had to find your compromise manually (usually through the tools and bounce reports other ISPs gave you), fix your stuff, and pray Google would accept your mail again in a couple days.
All that on top of their preference for marketing mail and lack of end user spam sensitivity flexibility. GMail is a bad tool that won by being free, easy to set up, and ubiquitously attached to everything else you do on the internet (and not being Microsoft in a particularly anti-MS period of time on the internet).
at one point earlier in corporate history we had to call our lawyer on the phone every time we sent him an e-mail because G Suite was helpfully sending all of our e-mail to spam despite the fact that we had, at that point, been working with him for several months
