They know what they are doing and using all their connections/power to survive. Its easier to do what is familiar than learn something new, especially for older employees that are ingrained in their routine/ways. It will be a huge challenge for them to learn new methods and tools that it will expose them and lose their advantage over newer team mates. At that point, they will lose their competitive edge and new joiners, younger employees will eventually push these types of older employees out. The alternative for them is to keep old ways and survive. Make knowledge sharing non existent, make sure no documentation exists and they will be the only source of contact for this info.
I don't blame the employees doing this, I blame the managers for letting this happen. A good manager will not make this happen and will force everyone to be equal and avoid these silos. There will of course be some employees that specializes on certain topics, but their processes should be documented and shared among the team in case new joiners were to learn these specialized preprocess or if a team mate would like to take on these new responsibilities.