The UK has a credibility gap with the rest of the EU, and its one that has dramatically weakened Teresa May’s bargaining hand in so many ways.
The EU27 suspect that the UK won’t really leave for two reasons.
1. It can’t actually leave. According to this argument, the practical barriers will eventually accumulate and drown out the threats that we hear from Brexiters whenever any practical scepticism is voiced, or when constitutional checks-and-balances (AKA “the enemies of the people”) are used.
2. It doesn’t want to leave, no matter what is says to the contrary. We’re not certain that the people really want to leave. Referendums are never a good way of working out what citizens want to happen, and June 2016 was hardly decisive. Moreover, from a continental perspective, many EU member states have a well-founded historical distrust of referendums and their results.