yes, blame the babylonians (i always do).
for a time, it was thought that the year had 360 days in it, which would make 12 months of 30 days each. note that 12 is 1/5 of 60, and 30 is 1/2 of 60, so these are "convenient" fractions in a base-60 number system.
alas, it took astronomers some time to realize that this estimation of a solar year was incorrect. we have been paying for it with various calendrical "corrections" ever since (with bizarre conventions of leap years, and mis-matched month lengths, not to mention the whole gregorian/julian snafu).
apparently, early philosophy literally thought that life was circles within circles, with everything lining up like some grand cosmic clock. the gods themselves created this order, and although such a notion may seem on the surface ludicrous, it persists in many of the common elements of language and thought (including, ironically, a great deal of relatively "modern" science).
early astronomers also thought that the lunar cycle and the solar cycle were synchronized (a quaint reminder of this is still evident on many old clock dials, and even some wrist-watches), which is how subdivisions of solar years came to be called "moons" (months).
which goes to show that an idea, even if totally wrong, may well outlive its usefulness, causing hair-pulling for young geometry students everywhere.