We are used to 2 liter bottles, so we still use them. We run 5ks because its been a standard distance to run for a long time. Other countries also do similar things, old habits die hard.

We use metric for science and medicine because the benefits of metric are much more pronounced for those use cases.

Honestly, using both really isnt that hard. Its only really an inconvenience if you aren't already used to it. We aren't changing it because we're getting along just fine the way things are, and there are much bigger problems to be solved.

See also: the 9mm and 5 grams in my pockets

want to fix an american car–hope you have both metric and “standard” wrenches

I will point out that with the singular and shining exception of lugnuts, at least this one has not been the case since at least the 1970's. All fasteners on current(ish) American cars are metric nowadays and have been for quite some time. I've never seen a single one that isn't on any car that's not old enough to qualify for historic plates.

This used to piss off the oldheads to no end back when I managed a hardware store because they would absolutely insist, sometimes literally screaming in my face about it, that their dang old good old boy red blooded American Ford that they just bought didn't have no Jap pinko metric bolts in it anywhere not nohow, and 100% of the time they were wrong. (This annoyed me only slightly less than the people who showed up needing a bolt, didn't know what it was, didn't bring the old one with them, and the only information they had was "I took it off with a 9/16 wrench." Hombre, the head size tells me absolutely nothing about the diameter, thread pitch, or length. Then they would claim that it's just a "standard" bolt, as if there's any such thing. Also, a 9/16" wrench will usually fairly easily remove a bolt with a 14mm head, so that really tells me nothing. Or 5/8" on 16mm. Etc.)

Harleys, however, take it as some kind of point of pride that they actually do use fractional inch fasteners everywhere.