Security is hard. There are many, many more compromises of small firms and self-maintained servers than of these big players, it's just that they don't get major media coverage in 99% of cases.
Take 100 people, and suppose 1 of them decides to stop using Gmail, replacing it with a custom setup. 99 decide to stick with Gmail. The 1 person who spent hours on a custom setup is more likely to leave a comment sharing their experience, tips and tricks, etc. The 99 won't have something noteworthy to post about.
End result is you see disproportionally more comments from people who do something drastic and unusual compared to ones who don't.
1) Google
2) Every government that has the power to compel Google to release your private information, from Chile's to the Cayman Islands
3) Every agent empowered by any of the governments from 2)
4) Every person and/or organisation that could be furnished with your data as part of some sort of "discovery" process by any of the agents listed in 3
(for the US: essentially anyone and everyone you've ever had any sort of commercial relationship with)
This means that if you, say, have a divorce case, expect your entire Gmail contents to be used against you by your significant other.
The problem is well explained here:
https://www.stangelawfirm.com/Articles/Facebook-Evidence-and...
This has nothing to do with Google. In a divorce case, you would be compelled to disclose your email, not Google, so using another provider wouldn't matter, because you would be compelled to turn over any emails there too.
(incidentally this is exactly why most corporations these days have a pretty short email retention policy, something like 3-6 months. As long as you delete the mails before a complaint, or at least before discovery is granted, there's nothing wrong with deleting them, even if it is to avoid them being used against you later. But there's other advantages too: it helps me get organized, and it prevents me from obsessing over things that have slipped so far down the priority list they'll never happen. Which is very soothing. Plus it breeds the good habit of not storing important things in your inbox)
I would like to point out that often the criticism is leveled that you'd only do this if you're guilty. Aside from that that is the "if you've got nothing to hide ..." argument against privacy, it comes with a lot of false assumptions. For instance, that such information will not spread, that laws won't change in bad ways you can't control, that you can trust law enforcement infinitely and indefinitely, and so on and so forth.
Also, a wise man once said: "In theory, theory and practice are the same. In practice however, ...". What you control is yours, and with proper security measures no power the police or justice system has can break that control. That's as it should be.