1) A lot of designers work at an agency at one point or another. The environment there encourages behavior very different than what startups need. E.g., long cycles vs rapid iteration, "final" deliverables vs incremental release, an emphasis on what clients want vs what users need, hopping from project to project vs really digging in on one thing, ability to specialize vs necessity of being an adequate generalist, high-church design vs getting one's hands dirty, and being hip vs being passionate.
2) One gets to be a really good designer by designing a lot of different things and keeping very current on trends. But startups require a long-term focus on a single product, which takes people out of the design game. That's a very hard sell.
At our startup we outsource individual pieces of design (e.g., logo creation, color and layout selection, coming up with a new look for something). But that ends up serving as raw material for the in-house activities. Most of the design ends up being done by the product manager, the engineers, or both collaboratively. That works for us because we do a lot of user testing and iterate frequently, and also because we know when to hand some piece of visual creation off to an expert.