Why such a short automatic log out on here?

If don't want to be logged out for long periods of time you need to check the 'Remember Me' box when you login. You also have to allow cookies, at least for this domain.

Goes for most domains. They will log you out as it clogs up server space and causes the system to run slow if there are a substantial amount of users on a website. Most time outs are 15 Minutes to avoid this. Except for larger servers such as Facebook. As there is millions of people to accommodate for.

There is also other reasons for log outs. The cookies are your computers website memory ;)