The problem is this: e-mail (SMTP/POP3/IMAP/what-have-you) is an ancient, simple protocol originally intended for sending plaintext messages in a trusted network. Using it for sending or receiving large amounts of binary data across today's Internet is a bolted-on hack, completely different from the original use case, and it performs rather miserably in this role.
When you attach a file to the e-mail, it gets base64-encoded, which increases its size by 1/3. Thus, your 1 GB file becomes another 300 MB larger; also, there is no built-in compression to the download protocol, thus no way to speed up the transfer (and in some cases (SMTP for sending,POP3 for receiving), even no way to resume a broken transfer - connection broke at 1.2 GB? Sorry, you need to re-transmit it all again). Moreover, SMTP is a store-and-forward protocol. Guess what? Yup, that 1.3 GB file needs to be copied across multiple servers; cue unbounded happiness from the mail server admins.
This was a problem in the 1990s, when there was no useful alternative (FTP? HTTP/1.0? Puh-leeze); but in the glorious year 2011, with various ways of seamlessly up/downloading data to/from the cloud (e.g. Dropbox, Ubuntu One, Amazon S3, to name the most known), the excuse of "there's no other useful way to do this" is not true any more.
Note also that not everyone is on a 100 Mbit link to the Internet - e.g. mobile and smartphone; not every mail client is capable of downloading only the headers (e.g. POP3 is still in much use), and not every user is willing to download the 20 inevitable "look at this funneh 1 GB video" e-mails per week that will appear (people will send as large files as the system will let them; and yes, there is something like FUP with most ISPs).
TL;DR: while it would be technically possible to do such things as e-mailing a 1GB file, it would also be technically possible to pound in a nail using a screwdriver - it's just not a good way to do it, as there are tools that are more suitable for such tasks.