Because there is seldom a dead cross between our caliber and the metric equiivalent. Some folks assume that since 40 cal. is also 10 mm that there are .25 mm per cal. and this won't work.
I think a big factor is that our caliber designations sometimes have little to do with the actual bore diameter as opposed to the metric system that is more closely aligned with bore diameter. For example the 32 acp that you mention has an actual bore diameter of .311-.312 and 38 cal. is .356-.357 where as 9mm is .355 and the list goes on.
Just not an area where things don't clearly convert ... you come close and that's about all.
If you only have time to do two things so-so, or one thing well ... do the one thing!