For pretty much the same reasons as I've already outlined, Factor. There's a 'v' in five but an 'f' in fifth, too.
As it happens, in Old English, twelve was written as twelf and twelfth as twelft. In the 14th century, the latter became twelfth to chime with the other 'th' endings...fifth, seventh etc. Twelve itself was spelt with a 'u or a 'w' in second-last place from the 12th...would you believe it?...century, but is recorded nowhere specifically with a 'v' before the late 14th century.