its easy to alter tyre widths to metric, but imagine the logistics of the diameter !! wheels have always been measured in inches, if the manufacturers suddenly decided to go metric( unless they used the metric equivelant of inches which would cause even more confusion) it would mean all tyre manufactures would have to come up with a complete set of new measurements for every tyre !!! would cost 10s of millions.
As a point of interest the plumbing trade mixes metric/imperial
As R1 said, there are metric tyres about, the old BMW 901's 525TD used to be fitted with TD230/55ZR390 tyres, so the rim size was 390mm!
All that happened is all the owners found out when they went to get the first set of tyres that it worked out cheaper to buy 4 new 16inch rims and a set of tyres to match rather than 4 390mm tyres.
(I think some of the early Austin metros came with metric wheels too, and everyone changed them too)