Why are the number of days in each month not the same for easy calculation?

We all know that a year has 12 months, which is 365 days. So how many days per month should be reasonable?

Later, when people compiled the calendar remained unchanged.

If calculated by 30 days, a year should be 7 months 30 days, 5 months 31 days. But now the number of days in each month doesn't count that way. Why is that?

According to records from a long time ago, there was a Roman Emperor who thought that February was not good, so he ordered to reduce one day of February to bring in July.

Later a Roman emperor said he was born in August, that month must also be added one day. So get back to February one more day. Therefore, February has only 28 days.

Later, when people compiled the calendar remained unchanged. So the current calendar has 7 months and 31 days; They are January, March, July, August, October, October and April 30 days; it is April, June, September and November. Particularly February has only 28 days or 29 days .

1. Why are Vietnamese and Chinese calendars different?
2. Why is there a leap day 2/29?