Because there are 365 days and they cannot be divided equally into twelve

It is pre determined
we can not help it

Originally, the months all had the same number of days-30, and the Egyptians added another 5 days for birthdays egyptian gods which adds up to 365 days. all these were based on the movement of the sun. however, when it came to the romans (they were superstitious), they made the number of days in each month an odd number (considered to be lucky) resulting in many confusion and out of line with the seasons. hence, Caesar adopted the solar year which amount to an average of 365.25 days. however, despite the regulations by pope Gregory XIII to correct an error by Caesar, the no. of days in each month continues to be in the way the romans had changed hence we have different no. of days for different months. and since we are using the solar calendar, the time taken for the sun to travel around the sun on its own axis, it is 365.25 days.