Why don’t Javascript object track the count of it’s key?

There is already a size object for JS Map class but for object there is no property like size. Currently I am using Object.keys function in a for loop just to find out the length. Since Object.keys is O(n) or even O(n logn) what is preventing browsers from implementing function like say Object.keyCount which gives the number of keys in the JS object with an internal counter. I am just curious why is it not still there?