A motor easily provides enough power to overcome its weight, and they wouldn't need assistance for the entire race, just an edge at key moments.
Think of the riders themselves as incredibly efficient batteries and motors - they can also recharge at 120g carb/hour. The motor itself is just deadweight over most of this process.
For much of the stages, the top guys are not doing much work, they spare their legs for the climbs. They will hide in the pack, doing only very light work drafting. If you could put a smallish battery able to recharge on flat / downhill sections and only provides a boost on the critical uphill parts, that would be a massive advantage.
Now it could definitely work if you only care about a single attack - not even a full climb, but maybe 200-300m where you want to distance your rivals, but the risk (not just of discovery, but of malfunction, etc) just doesn't seem to be worth it. There's domestiques for a reason.