Today is Washington's Birthday. It is NOT President's Day. The other 43 don't get a day dedicated to them. Maybe we used to celebrate Lincoln's and Washington's birthdays at the same time, but the official federal designation for today is Washington's Birthday. By celebrating it on the 3rd Monday of the month means that the day we celebrate Washington's Birthday can NEVER fall on his actual birthday, but I'm sure George would appreciate a long weekend.
Some President's were good enough to deserve a celebration. Regardless, we don't celebrate their birthdays. Including the others into one day would include both Harding and Grant. Either is an excellent example of why the day is not dedicated to them. It isn't that Washington is my favorite President (Bully for you if you can figure out which one was) but it is a day off work. While some folks only care that the banks are closed and the mail won't run, it is important to know why.
Just remember, President's Day is a sale, Washington's Birthday is a holiday.