[sfifs]

You would do on-prem and not cloud if your potential legal adversaries included the government because they would then have to come and take your emails from you with a warrant vs. being able to silently take it from cloud providers and compelling them to not inform you.

For any reasonably sized multinational, governments are potential legal adverseries.. and so they avoid keeping mail servers and financial transaction data in the cloud

[uiri]

they would then have to come and take your emails from you with a warrant vs. being able to silently take it from cloud providers and compelling them to not inform you.

What if they just insert a box upstream of your connection via your ISP?

[fictioncircle]

It doesn't collect emails routed within the LAN/VPN. You can secure same domain email address communication to prevent such mitm.