Keep in mind the French government was going through a shitload of changes at this time. They collapsed mid-crisis more or less and significant changes occurred. The Germanys couldn't be guaranteed who they would be negotiating with. The British also had internal strife from people like Churchill which made it more difficult at times for Chamberlain to pressure the French.

This is on top of the populations being far and away pro an alliance with the USSR. So Germany can't trust that Britain can fully just control French policy, or at least is not privy to how cucked the French really were to Britain and so worried immensely about the French government's gestures towards the Soviets. Remember things got so far as Romania offering the Soviets their airspace to protect Czechoslovakia. Germany could've been dealing with a real alliance any second now as far as they knew. They picked what is called the phony war over risking that