Because being born isn’t a choice and turning children into “illegal aliens” simply for coming into this world is immoral. If criminalizing being born inside a particular country isn’t a human rights issue, I don’t know what is. Regardless, your numbers are false. We are talking about around 250,000 a year, not millions. Ninety percent of those are born to parents that have been in the United States for at least 2 years according to research done by PEW, including many on work or other visas. Hordes of expectant mothers are not crossing the border during their final months of pregnancy.

https://www.migrationpolicy.org/news/repealing-birthright-citizenship-unintended-consequences