Emails are sent over different servers, sometimes several, before being delivered. Each of them has to push the attachment forward. Making attachment sizes too permissive opens the door for a pretty neat DDoS vector.

Most EMail servers do not accept more than 10MB, and considering the file sizes of your typical PDF, Word Document or photo, that seems sensible.

The problem with email attachments is that they're stored on the recipients inbox forever unless the message is specifically deleted. If the attachment limit is very large, you could fill up someone's inbox with a couple of messages.

No host in their right mind would accept such large messages.