I'm mestizo, but look white?

You're a mestizo of predominantly European ancestry, hence you get confused for European ethnicities. Plus, being a Latin American mestizo, your white ancestry is almost certainly Spanish, hence you get pegged for very similar ethnicities like Portuguese and Italian, also Southern Europeans like the Spaniards. There really aren't many Spaniards or Spanish-descended people at all in the U.S., so a Spaniard or someone of mostly Spanish descent will probably get confused most for being Italian, Portuguese, Greek, Jewish, or some other Mediterranean white ethnic group that's more represented in the U.S.