Well, how do you get the starter rope through? Unless you put a clothes line like system in each conduit. That's a lot of extra hardware/cost per conduit, even though it's only used on the rare occasions when something is changed.
Special machines are an upgrade though. They used to use ferrets. No kidding. Early 1900's, they'd train ferrets to crawl through underground conduits to run cable. Tie a string on and send it through.