PS3 doesn't do much in the background, checks for network connection status, updates online stuff like friends, messages, etc. and plays a game from a dedicated storage medium.
A PC on the other hand, is checking network connectivity, checking hardware status (S.M.A.R.T.), checking for DMAs and IRQs and checking each hardware device attached to all PCI, PCI-X, AGP, etc. connections, controlling a USB bus and searching for devices on that, doing whatever background tasks you have open, plus anything any other users have open, plus other background stuff as other user accounts (defrag, computer browser, dns stuff, network stuff, loads of things)... And I could go on. Your PC kernel has a very complex security structure built into it with passwords and hashing and whatnot, the PS3 doesn't have such a feature and is limited to just running things with either full permissions or limited permissions (what the other OS function did), and it all runs as the equilivent as one user.
I'm quite sure a PS3 does plenty of those things as well. It's still a computer with components that have to communicate with each other, USB connections for controllers or memory cards, wireless networking and connections to remote controllers, registering a lot of input continuously. There are also said security features, while likely not as "complex", still manage accounts with credit card details, passwords,, online stores with purchases, achievements et cetera.
Not saying you're wrong, but a console is a computer, running an operating system.