Why still using the imperial system?

There are (huge) costs to retooling production to move from imperial to metric. Even if a company wanted to make that move they'd have to transition in phases and will likely end up with additional equipment to maintain. There's also significant training for workers (who will likely commit errors in the beginning) which will impact production. And what happens to the old equipment? I'd guess a significant portion of that would end up getting scrapped and landfilled.