My question is why the integration of other mail-accounts via IMAP is not possible?

Background is the well-known fact that IMAP is by far better and more reliable than pop3.

I would assume, that this is not just a missing functionality, but there is a deeper technical or even a license reason.

Could you please give reasons?

Examples: 

gmail in web-browser also supports other mail-accounts only via pop3, not via IMAP

In contrast, the email app on android or the gmail app on android supports IMAP.

Thanks for your investigation to shed light on this!