I'm not sure it's hard, is there evidence? Personally I don't think there's any valuable benefit, certainly not worth all the resulting disruption, and wouldn't advocate for it for that reason.
I've gotten into wood working the last few years and I've found that I'm significantly less likely to make a measurement mistake when I use metric. It seems like nearly all my measurement mistakes are when I did something in imperial.
So why don't I use metric all the time? Because materials are usually sold in imperial units.
Suppose you have a 30 cm piece and you need to evenly space 5 3.5 cm-wide wood planks on the 30cm piece. The decimal calculation will take you to a resolution you likely can't measure confidently (0.25 mm)
So US companies cannot export surplus and will need new tooling when they want to export. That makes it much more difficult to expand into international markets.
E.g. Japan use metric: https://www.kubota-chemix.co.jp/dcms_media/other/20161226_en...