"Why is this number considered to be the largest ??"
This is the largest NAMED number. Obviously, there are an infinite amount of numbers larger.

Even if our use of the concept of numbers began with counting, mathematics (which encompasses not only numbers but the relationships between abstract quantities like time and gravity) is a language. It's the language of our reality. In English, we can certainly use words to express concepts that don't physically exist in our Universe, and we can describe imaginary objects that don't (or even can't) exist physically. By your logic, the following words wouldn't exist for me to write: the orange-colored Beaver Queen of donuts.
Math is no different. It's a language fully equipped to describe any numerical concept, whether or not it could exist in our Universe.