This. Worrying about people hacking through Steam to steal your details is a bit like the people who used to go into police stations to confess when their pc popped up with 'This Computer has preformed an illegal operation..etc'
The majority of people who got hacked using services like Steam made the following mistakes.
They use the same username and password for everything (A mistake i hear is made a lot)
They replied to emails claiming to be from Steam requesting your username and password (Steam will never email you to request either)
They did something stupid like share their password.
They broke one of Steams rules and had their account terminated, but rather than admit it, they cried about being hacked.
You forgot to mention.
They visit fake sites.
They click on email links (Taking them to keyloggers)
They don't care about internet security/Anti Virus.
Most common case of account theft is due to clicking on links in emails sent from fake accounts, that links to keyloggers.
As an example. "From - ***Fake email/website*** - Hi, we are investigating your account and have found out that you have breached our EULA/TOS and need you to send us your account information by clicking on this link and accepting blablabla" The reciepient clicks the link, accepts whatever he/she is told to accept and to click, and their account info and personal info is being transmitted to the evildoers on the other side of the world (In most cases china/USA)
It doesn't require you to be a rocket scientist in order to keep your information and your computer protected. It does however, require a bit of awareness and a tint of personal intelligence, not to mention the ability to think logically.