Granted, but they usually have the option to sell at many points along the way.
Think about an alternative financial instrument. What if you could invest $X in a public company (and get a higher rate of return because of the higher risk) but you lose the ability to sell until five years have passed?
This is one kind of option that might be worth trying. I want to see more mechanisms promoting long-term investment in our public markets. (Bonds are not identical to what I described BTW.)
The lack of these kinds of options (sure, bonds exist but I've not noticed them used with any notable significance in public companies) is why people talk about the short term quarterly focus of Wall Street.