There are a few reasons: 1) having a single attaching point for the wipers makes designing stuff around it more simple 2) accurate curved tracks that match wi

There are a few reasons: 1) having a single attaching point for the wipers makes designing stuff around it more simple 2) accurate curved tracks that match windscreens are hard to manufacture, secure and keep in shape 3) Keeping constant force for all parts of the windscreen is much harder with a track (high and low spots, change the window screen, the track needs adjusting too) 4) standard bearings are much more simple to seal 5) linear bearing as a pain to seal 6) tracks are much harder to seal against the weather 7) tracks need more lubrication