...Because there are sitting members of Congress with the names "Donald", "Joe", and "Harris", and a former president with the name "Clinton"?
Ask yourself why people call George W. Bush "Dubya".
It has to do with how candidates choose to brand themselves.
Additionally, people often say "Trump", "Obama", "Biden". Saying "Clinton" is somewhat ambiguous (Hillary or Bill?). "Harris" could work, but it is definitely a more common name than Kamala.
Additionally there are other women in power who do get referred by their full names. People don't talk about "Marjorie", they use "Marjorie Taylor-Green". They don't talk about Ruth (Bader-Ginsburg) or Diane (Feinstein) or Janet (Yellen) or Margaret (Thatcher).
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)
Trump, Obama, Biden, Hilary Clinton, Kamala Harris (full name).
But interestingly, I call Jimmy Carter the full name, too. Can’t bring myself to use just “Carter”.
Though, why does the US culture so insist on taking the husband’s family name? It kinda feels like “Hilary” is the actual name of the person, and Clinton is imposed.
It doesn't really anymore. But it did when the Clintons got married several decades ago.