Because a library in a programming language would be madly inconvenient? I can't see something like this being pleasant to use:

  document(
    paragraph("blah blah blah"),
    math("sum_(i=1)^n i = (n (n+1))/2"),
    paragraph("foo bar")
  )

Also, I'd imagine being a DSL and having a dedicated compiler is what let's Typst have such great incremental compilation (it's actually instant).