You are comparing apples and oranges, at least a bit. The latex equivalent is

  \documentclass{article}
  \title{Natural selection in the Health and Retirement Study}
  \author{XXX}
  \date{\today}
  \begin{document}
  
  \begin{abstract}
      I investigate natural selection on polygenic scores
      in the contemporary US, using the Health and Retirement       
      Study. Results
      partially support the economic theory of fertility as
      an explanation for natural selection: among both white 
      and black respondents,
      scores which correlate negatively (positively) with education are
      selected for (against). Selection coefficients are
      larger among low-income
      and unmarried parents, but not among younger parents or those with less 
      education. I also estimate effect sizes corrected for noise in the 
      polygenic scores.
  \end{abstract}
  ...
  \end{document}

Everything else you have there in your preamble is about either adding capabilities or changing formatting, you don't show how that is achieved in the other markdown.

I think I get your point, but in practice that part doesn't really get in the way, and if you are doing the same thing over and over (e.g. for the same publication) it's just a template anyway.

I don't love Tex/Latex, but most of the other markdown comparisons that emphasize "it's simpler" are because they can't do as much. Which is fine until you need some of that capability.