But that's not the real reason. The reason is they are afraid because the experienced person can see through bullshit better. They are harder to fool and brainwash to work for free. With stuff like "There won't be a paycheck next month, but don't worry we are turning corner, once we exit you'll all drive Ferraris!", "We are changing the world, just need to finalize this feature by next week, so let's all work a little harder (and by all I mean you all)". Someone with experience will say something "Thanks but no thanks, I've heard this before" a new college graduate will say "Yay! Let's do it, I'll impress everyone with how little sleep I can get each night".
And I am not being too hyperbolic here. I've heard from owners before confessing that hey love hiring college grads because they can drive them hard, they are full of enthusiasm and are not "tainted" by other corporations.
Another friend from another startup, confessed that they like those take-home interview problems. Stuff like "Implement a distributed database over the weekend and show us the code on Monday during the interview". The reason is because they can get the really desperate and dedicated people willing to invest that much time into projects. As when crunch time comes, they'll need those "skills".