Because the RCC is one particular denomination of many within Catholicism.
There are various ancient Rites within the Catholic Church, including the Latin or Roman Rite, which is the predominant Rite in America.
The Rites vary in their methods of celebrating the sacraments, although the essence of the sacraments remains the same across all Rites.
And all Rites are obedient to the pope and the Church Magisterium.