Because 3rd-party-users like other maps or routing engines do not update their data on daily or weekly base but on monthly or even yearly base. Today and maybe the next few weeks there will be the road closed on Pontdriesch but in Juli it could be reopened and instead the Pontstraße at Kreuzherrenstraße will be closed. Sure, you can change that in OSM (which is a geo database not a map by the way), but it will take time until 3rd-party-users will know about it. Time in which construction work progresses. It is better to tag the whole road for the construction period with an approbiate tag (like "vehicle=destination") so that through traffic will be redirected. Residents know about the closing and theire visitors must be informed by them. That could not be handled by OSM.