1. They crave novelty. If you don't have enough, they'll get bored and wander off.
2. There are not many of them.
3. Unsurprisingly given #2, they're $$$.
But if you can get 'em, get 'em.
* get a relatively small number of really great people. This is how you prioritize flexibility.
* get a larger number of people (who don't have to be as good) and build a bunch of structure around them.