Because you'll forfeit your reward or upside for working hard on executing your idea, worse if it's successful where Google owns your IP and starts productizing it you've killed your opportunity to go start your own company around it.
Your pay remains the same regardless. If you're not interested in any financial gain, sure it can be more enjoyable to spend your work hours on your own idea inside Google's cushy offices.
That's a strange assumption. I would think that any employee who pioneers a successful idea will land a leading role in the subsequent product development team and a bonus at least. Sure, the reward isn't as high as if you spun off your own startup but it won't be nothing.
