Because that is enshrined in the Constitution [1] and amendments are very very rare to happen [2] even in a time when politicians were actually interested in running the country vs just "owning" the opposing side.
And there's many things that more desperately need constitutional reform than getting rid of "anchor babies" - FPTP, Electoral College, executive orders, or actually enforce the separation between state and religion to name a few.
[1] https://constitution.congress.gov/browse/essay/amdt14-S1-1-1...
[2] https://en.wikipedia.org/wiki/List_of_amendments_to_the_Cons...
