OEM's have the advantage of assembling everything within their specs in a clean controlled environment...
No matter how high the quality of an aftermarket part is, it's ALWAYS being installed in a dusty, grimy shop by hands covered with multiple solvents and petroleum products; which may or may not be compatible or harmful (Hand degreaser, used oil, break cleaner etc.)... but even if compatible they do a good job of trapping grit and grime and spreading it to new freshly machined bearings, and soft rubber seals etc.
If that doesn't kill em' being wacked in with a BF hammer and a socket/Tubing used for a drift, or having bolts torqued down to the torque specification of: "good n' tight usin' my breaker bar and s*it pipe cheater" will.