Probably because Renault wants "road relevance" even though it won't be. Mercedes and Cosworth presumably want the show as that's what keeps the punters happy. No F1 fan cares if the F1 cars are road relevant. They haven't been as yet, so I don't see why they should start trying.
Road relevance will only start when they have 110hp, airbags, central locking and a touchscreen 'entertainment centre'. And I don't think manufacturers need much help making them as they are, and ABS, Traction Control, Stability Control, Power Steering, Fuel Economy et al are all doing just fine (if not better) without F1 anyway (because F1 keeps the costs high, whereas road car production is about making it ultra reliable (not 200 mile reliable), cheap and user (or mechanic) friendly. F1 cannot and will never help in those regards.
Sure, some things filter down that have been used in F1. But stuff filters down from the Space Shuttle program and crazy stuff like that, but I don't hear NASA asking for more road car relevance to help their projects.