Part of it might be that Caesar can't use doctors and tech outside of very specific situations because it puts the whole structure of the Legion at risk. Look at who makes up Caesars army; a bunch of uneducated tribals who have never heard of Ancient Rome and really do think Caesar is the son of Mars and a God. Having them come into too much contact with civilization to kidnap a doctor, forage for parts for an autodoc, or find a machine shop to forge the replacement part for the howitzer makes them more likely to learn that Caesar is full of shit and they don't need to listen to him. Look what happened when Ulysses explored too much of the world, he left the Legion seeing how unsustainable they were. According to Sawyer one of the reasons he tried to kill Graham was because Graham was an educated man from civilization, knew Caesar was making shit up, and Caesar saw him as a political threat because he knew the truth. He can have the Courier do all this because the Courier is from civilization, knows its all a lie, but for some reason is working for the Legion so go enough for Caesar.
He has to keep his army as a whole ignorant and fearing of progress otherwise they will eventually learn he isn't a God, just a guy who read a lot of books and turn on him. His inner circle can have more contact with the civilized world because he knows they're loyal and fanatical, thinking nothing of their leader hypocritically using modern medicine and authorizing the use of more modern weapons. He can keep his closest followers under his thumb but if too much of the rank and file got too smart his ass would find itself lashed to a cross by his bloodthirsty goons.
Its kinda odd that the game shows that Lanius is actually intelligent, maybe even a little wiser than his boss because he can be persuaded to see how foolish it is to keep trying to take the dam, but the endings with him as the new leader of the Legion make him out to be a total idiot who does the stupidest thing possible. Maybe hes only good at warfare and not actually ruling a country. Or hes just a bloodthirsty butcher who gets off to mindless slaughter, whichever.