I realised for centuries that Santa Claus didn't bring
You have inverted 'clauses'...but gratefully, not 'Clauses'.
1. Correct: "I realised that for centuries, Santa Claus didn't bring all those native Americans in the Americas presents on Christmas eve. Why would he do this if he's not racists."
2. Should be 'Native Americans', not 'native Americans'. The latter would include all Americans born in the U.S.A. The former (your intent) would be the descendents of those peoples who crossed over the Bering land bridge. Also, in this context, the "Native Americans born in America" is superfluous, unless you meant "native (and Native) Americans born abroad" which would have been stylistically and grammatically better.
Why would he do this if he's not racists.
Not 'racists', 'racist'. Although quite "large", Santa Claus is singular, not plural.
4. Merry Christmas, and a very happy, healthy and grammatically correct new year!
Jafo would have been meaner (hope you're laughing)