Why do white people get angry about mexicans coming into america?

when this land is more of theirs than it is theirs or anyone elses? yet they get angry about mexicans coming here illegally even though mexicans settled on the west coast of america way before whites even came onto this land. plus most mexicans are also part native american which means they are indigenous to the continent. yet whites still dont want them in their own country. why do you think this is?