Comment by TeMPOraL

Because e-mail is naturally self-replicating and not bound to organizational boundaries, in ways chat isn't.

Chat messages tend to exist in one place only (vendors' servers), with maybe a transient local copy that gets wiped over time, or "for privacy reasons" (like Messenger switching to E2EE, effectively wiping cached history on any device that went through the transition). Chat message is an object, it's designed to exist in a single place, and everything else is a pointer to it, or a transient cache.

E-mails, in contrast, are always copied in full. You send an e-mail to me, you retain an independent copy, I get an independent copy, and a bunch of servers in between us keep an independent copy too, even if briefly. I forward your e-mail somewhere, more people and servers get their copies. I reply back to you, more independent copies, that also quote the previous messages, embedding even more copies that are even more independent. This makes it very similar to paper correspondence (particularly when photocopy machines are involved), i.e. impossible for a single party to unilaterally eradicate in practice.

And then chat vendors implement silly features like ability to retroactively unsend a message, force-deleting it from recipients' devices too (it may still exist in backups, but vendors refuse to let you access those, even with a GDPR request). In e-mail land, that's fundamentally not possible.

(Microsoft tried to bolt it onto their corporate e-mail software, but it only works in Outlook/Exchange land, and it's easy to disable (at least was, in OG Desktop Outlook - not the still broken New Outlook Desktop Web App). I discovered this when I once saw an e-mail I was reading suddenly disappear from my Outlook, which prompted me to find the right setting to disable honoring unsend requests.)

So, come discovery time, critical chat history may turn out impossible to find, and any deeper search will require forcing cooperation of the chat operator. E-mails, on the other hand, tend to turn up, because someone, somewhere, almost certainly has a copy.