Because a game calculation can't just be split up in equal pieces. That's the magic behind multi-core support, spreading out the calculations. Say one core is rendering the bulk of the game, another core is doing the physics. The core doing the physics, which is a lighter calculation only has to serve its calculation when the main core needs it. So it may sit idle until the game asks for a physics calculation and could potentially see only 30% use.
Same with every other 'split' in calculation, it needs to be calculated and served in the correct order and as I mentioned, it's a lot of work to split the bulk of a primary calculation up into a bunch of pieces because the calculation also needs to come back in one piece at the end.
That's why the upcoming generation is interesting, developers having to work with 8 cores. PS3 developers already showed they could stretch out a lot of calculations and make good use of the additional core support. Also the reason why you rarely ever see a game achieving perfect use across all cores - A 90% - 40% - 20% - 10% across the core use isn't out of the ordinary ( Random numbers ).