Because while a lot of Americans support a lot of left wing positions, there are no major left wing parties, and a very small number of politicians who run for national or statewide office who actually take action to further left wing policies. There's Bernie Sanders, who isn't a member of a large party. AOC, and a few others qualify, but being a small proportion of those running, they're a small proportion of those elected, and have relatively little actual influence.

Ideas neither major party supports are basically impossible to see happen.