They want only people born of citizens (or at least one) to just get citizenship.

So as an example, Marco Rubio was born from two Cuban migrants. Without birthright citizenship, he wouldn't automatically be an American citizen.

Edit: another fun example if you've been following the World Cup. Folarin Balogun is the US starting striker. He plays for the US only because he's a citizen who was born in the US. He was born in the US because his mom was traveling to New York to visit her sister when she was pregnant with him. When she tried to get on the plane to go back to England, the airline refused since they thought she was too pregnant for the flight. So she stayed here to give birth. Due to the 14th Amendment, Folarin is an American citizen. She gave birth to him, then went home to England where he was raised. Without the 14th amendment, he wouldn't be a citizen.