If company Y is willing to pay 50% more to hire someone from company X, it's often because they want to bring in someone with external experience. A person already working at company X has that to offer to company Y, but not to their current company.

If company X isn't looking for external experience (because they figure they already have that experience in their senior management, other team members, etc), then they'll be happy to let the person leave for company Y and hire some new entry level / fresh grad / minimally qualified person for the position and start over.