Because the United States is exactly that, a union of fifty states. Each state gets representation in the federal government, each state has a say in who the president will be. Dump the EC and the low population states have no say in who's president. The same thing happens in the senate, each state gets two, doesn't matter what the population is.
It's how the republic was set up, and it's worked for near 250 years. That's a good system. It's produced what's arguably the most successful nation on the planet.