I agree, including other guided transit (trams, urban gondolas...) on the metro layer would greatly improve public transit routing.
But bus routes are a harder problem to integrate, especially in larger cities. In Paris, there are more than 200 bus routes (more than 1000 including the greater suburbs), map readability would take a probably big hit if they were displayed on the map, or at least it would require a lot of care to do it right. A some larger cities also have a night service for busses, with routes differing from the day busses, handling those properly is also an issue.
That's actually one of the few areas where I really like the way OsmAnd approached the UI. When you tap a bus stop, it shows you which bus lines stop there, and only after you tap one of the lines do you actually see the route for that particular line, and it has buttons to focus the next and previous stops.
But bus routes are a harder problem to integrate, especially in larger cities. In Paris, there are more than 200 bus routes (more than 1000 including the greater suburbs), map readability would take a probably big hit if they were displayed on the map, or at least it would require a lot of care to do it right. A some larger cities also have a night service for busses, with routes differing from the day busses, handling those properly is also an issue.