Bernie Sanders is an immediate and obvious counterexample to your assertion that people only do this for Kamala Harris and Hillary Clinton. There's also Jeb Bush, Pete Buttigieg, Marco Rubio, and many others.

The exclusion of these fairly obvious counterexamples suggests that you are engaging in a form of confirmation bias.

There is reason to believe that people are sexist toward female politicians, but the first-name basis of Kamala and Hillary is not a strong piece of evidence, given that we do that for the aforementioned male politicians (and don't do it for a lot of other female politicians, such as Elizabeth Warren, Sarah Palin and AOC)