Maybe they are under contract not to pass information to third parties or maybe the company policy is to not let internal email off the network.
That you don't understand it is likely from the perspective of an individual, possibly a private user. For those applications you can't beat the cloud. For business use every business needs to weigh their own needs.
Even then though, many business think they need to have their own server when they really don't and vice-versa.
Sure, but those are rare and usually include enough budget to include sysadmins and definitely enough to buy ECC memory.
For anyone weighing whether ECC is worth it, they are wasting time managing their email server.
Those are not rare at all. Every lawyers office has this problem, every journalist, every banker, every insurance company, every notary public, every administration and so on.
No, but they are contractually required to keep their customers (and their own) data confidential. And that can lead to them deciding to run their own mailservers as well as other infrastructure. Whether that's a good decision or not is another matter, that mostly depends on execution.
If you are using the cloud for email they can usually see all of your activity. Email is also how you typically reset passwords. It can expose corporate secrets.
In practice, nobody encrypts their email. And even if they do, the cloud still gets all the metadata.
Running your own trades the above issues for other issues, but depending on your priorities and fears it might be worth doing.
Conventional legal mechanisms against your home server cabinet can be handled via full disk encryption and a reed switch on your cabinet door connected to your power strip.