From Loot Ninja, Valve Game Designer Tom Leonard had this to say:
The PC and the 360 are just more straightforward. We can focus on what we want to do, which is make game experiences, instead of sweating bullets over obscure architectural decisions they make with their platform. [...] I didn’t come into this business in the 90s because of some technical fetish. I came in because I wanted to give people experiences that made them have fun.
And, could Gabe Newell's status as a former employee of Microsoft be another reason?
Its not just cell. Consoles are designed diffrently then PC's. For PC's, you just code in C++ and let the compiler do its thing. For consoles, you have to worry about Registers, memory managment, number of bits to use (8 bit Int, 16 bit Int, etc) to save space in main memory, etc.
It's not a conspiracy because somebody used to work for MS, and it's pretty well known that PS3 is more difficult to program for than 360 and especially PC. Also, even if you don't understand much about programming it's kind of plain to see that if one company, Microsoft, develops both a console (360) and the main PC gaming API (DirectX) that developing back and forth from one to the other should be much easier that it is for a completely separate system developed by a completely different company.
One of the keys to Valve's success has been that they don't constantly go around creating brand new engines or games from scratch. This allows them to focus more on developing the actual games rather than new engines and makes there games run scale better to a larger scope of hardware configurations. If you look at their model it would go very much against what they do to try and develop from the ground up for a system which is only viable for a few years and then have to start over again when the next system comes out.