It is and it isn't. When you work with both systems, not only you remember conversions, but what the imperial/metric size equates to imperial and metric materials. So for example, 8 foot by 4 foot plywood/MDF is 2440mm x 1220mm in metric, which is exact. But if you order an 8 foot x 4 foot plasterboard in the UK, it comes as 2400mm x 1200mm, but we still say an 8b4 sheet. They come as 9mm, 12.5mm, and 15mm thick depending on you stud centres.
4b2 rough sawn wood is exactly 4" by 2". But if you buy 4b2 CLS, you will find it's planed down below that imperial size.
We work with both and have no problems, I get the sense most yanks struggle. I've even measured a Douglas Fir lintel, it had to be in imperial because of the old building (probably older than America). My tape measure went to 8m, so it something like 7m and 7inch. I just go by whatever hits easier on the tape measure. I don't have to be stupid and anal to stick to one or the other. Some retards just stick to one and claim how brilliant it is. But I can't fix stoopid.