The biggest reason why MTB are imperial cause it doesn’t really matter if it’s a couple millimetres out like road tyres as the clearance on road bike are tight enough to the point 1 millimetres make a big difference.
2.1 inches can be either 2.05 or 2.15 and it will still fit fine.
Road and gravel bikes = metric
MTB = imperial, unless you're measuring suspension travel, handlebar width, seatpost diameter...
Ah fuck it, it makes no sense