My employer does have corporate travel concierge who can actually do this with minimal input. But people do not want that. They think about aspects like, if I have a layover at XYZ, would the vicinity of XYZ be a good place for a few hours of sightseeing, given my own travel preferences and previous sightseeing destinations; and would I still want it if it is an overnight layover at XYZ. Or things like, if I fly via this route, I could squeeze another paid day with zero work expectations while I could rest on the plane / in lounges. Or things like, this particular airline has this particular type of aircraft with far better seats than that particular airline, so I would be happier even if the flight is longer.
Needless to say, all colleagues that I know of (myself included) prefer to book corporate travel themselves, even if from the company's perspective, spending a few hours to do it is much more expensive than having the concierge spend a few hours to do it.