Busses are very slow compared to any subway. Even with dedicated lanes (which is costly and unpopular) they still need to go around obstacles like buildings, follow the roads, yield to pedestrians, pull into stops, etc. Speed limits need to be obeyed as well.

Compare that to a subway that just goes below most obstacles in a straight line, travels at >80km/h and usually has no places to yield whatsoever. Fast travel in a predictable way, independent of traffic, weather, whatever. Maybe rush hour will make getting into the subway a little slower, but that's about it.

Subway is far superior to everything, including trams and trains for urban environments. It is expensive to build however, which is why in Europe it is usually supplemented with trams and busses for outlying routes or to fill the gaps between tunnels. Flexibility is rather a minus actually, because it leads to bus routes being subject to "snaking" through neighborhoods to satisfy the occasional political whim. Planted 5 trees and a bench? Call it "Mayour Mueller Park", add a bus stop with a 5min detour. Lather, rinse, repeat. Subways are somewhat immune to that.