Why does the atomic mass of an atom equal the weight of 1 mol of that atom in grams?
And you might just as well ask why there are 12 eggs in a dozen eggs......
A dozen is a convenient number for counting, because we can use our thumb to mark out a dozen with the tips and joints of the four fingers on each hand.
On the other hand, a mole contains
Why should we use such an absurdly large number? Because
And in every chemistry exam you should be given a copy of the Periodic Table, which tells you the molar mass of each known element, as well as its atomic number. It is up to you to learn how to use the Table effectively. Good luck.
I have said much the same thing here, and in the links.
Note that on the Table, the atomic mass of carbon is quoted as