Why fediverse apps always need a subdomain?

Hi there o/
(I’m french, but let’s try in english for accessibility. Therefore, sorry for language issues :wink:)

It seems that all the available apps using the ActivityPub/federation mechanism (Mastodon, Pleroma, Plume, etc.) require a dedicated domain, and I wonder if there’s a particular technical reason for that?

I know that YunoHost packagers usually follow the developers’ recommendations for that, but it’s strange that all the developers of such apps did the same recommendation, so… does the ActivityPub protocol need that?

(Moreover, I would like to be known on the fediverse with the classical login@maindomain, as for mails and XMPP (well, probably @login@maindomain here), so if you know any available app that can be installed without blocking all the main domain, please let me know; but I started this post mainly for the technical explanation, being curious)