Because I'm going to be in the game for at least another 30 years and who knows how many opportunities to work together again might exist.
Also, the idea that an employee is a specific cog that will indefinitely fit into the mechanism in the same way is an outdated relic of using a machine as a metaphor for a firm.
While some employees reach limits, others will continue to evolve, particularly those at the beginning of their career. I have not ever been in a position where I prefer a less ambitious/promising candidate out of fear they will leave after a few years, though I know some people in HR look at the world that way.
