Why aren’t train car wheel sets (bogies/trucks) more standardized?

Why are there so many types of trucks/bogies for train cars, even for similar types of train cars?

For example, M-8 train cars used by Metro-North and the Long Island Rail Road around New York City use trucks that rattle and squeak a lot. Why not simply use trucks that worked for a previous model of those railroads’ similar cars?

New Amtrak cars also have trucks that differ from ones than Amfleet cars used; why not just use the same ones that Amfleet use?