Good question!

The primary reason is database size. It's a good idea to maintain a reasonable DB size. Too big makes it hard to manage and greatly affects which hosting companies can/will handle it if you have to move servers at some point.

To try to keep things in check I set some limits when the forum was created, which I've recently increase.
Max size 1680x1050
Max attachment size 256kb
maximum per post 3
etc

The secondary reason, and the one most user will notice, is to keep file sizes/page load times in check. Not everyone who visits has broadband. Allowing the posting of images with unlimited size could really hose someone on dialup!