Why is it often said that 10% of office workers do 90% of the work?

It's an overstatement. But refers to a fact that there are a few who do much more than the rest.

Because they are carrying the weight of loafers who get paid to look cute and waste time.

Smart but lazy employees figure out how to exploit their coworkers while projecting an image of "busy."

Roles and responsibilities vary. Work processes might lead to a busier than usual time for each. Seeing What We Believe Sessions could ensure that big picture views are consistent.

Didn’t know this was often said

I have know idea what that means but office workers work hard and use there time wisely

Because it is true. Somehow the other 90% can get away with doing less and knowing less. Generally this equates to who you know in management.

When all staff are not equally held accountable for their actions and inactions by leadership

Some folks just try to get by while others have a good work ethic and morals.