How can it take 6 years to make levels that have Nothing in them besides a couple head crabs + a switch to open a door?
Where do you think they got the models for those head crabs? Or the textures to put on the head crabs?
What I'm getting at is, game development has become extremely expensive and time-consuming, because many people don't like to play games that don't have state-of-the-art graphics. A lot of those same people don't care if the game only lasts 6 hours as a result. Valve has got this all figured out.