Many reasons why..one, mechanics would much rather change a pre-made control arm rather than have to hold and position the control arm on a hydraulic press and attempt to press in the bushing (its not easy to do and do right).
Removing the control arm is not the problem at all, however sometimes the bolts rust to the bushing collars and that is fun, have to heat the bolts up with a torch which usually results in a melted bushing and small fires, quite the pain in the ass
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