Why do we only have a leap year every 7 years?

We do not have a leap year every 7 years. It is every 4 years. It is possible that in a sequence of 7 years, that there is only one leap year in the middle of it. For example 2000 and 2008 were leap years, so in the intervening 7 years, there was only one leap year:

2001

2002

2003

2004 was a leap year.

2005

2006

2007

In order keep the calendar years synchronized there has to be a extra day added to February every 4 years. The other years that are not leap year are called common years.