Because lots of things work this way. For example compilers built on LLV uses an intermediate language and Python uses byte code.
I suspect some html to pdf tools go through postScript.
Of course Latex gives you fine control to hand tune the engine…but that doesn’t seem like what the OP is looking for.
Theoretically you can drive nails with a 22 caliber blank cartridge without making the “call” through a nail gun. But you won’t finish laying shingles as quickly and easily…
Or to put it another way, there’s a reason assemblers are almost always better than machine code and compilers are almost always better than assemblers for the ends people care about.
I mean why use Latex at all when you could write your own typesetting language? Maybe because you are not a knuth.
I mean everything has dependencies (some of the solutions elsewhere require Chrome and other common solutions require the JVM). At least Pandoc is GPL.