Weird, isn't it? Britain and France had every chance to attack, yet didn't. Why is that? Because it was deliberate.

Ww2 was a huge optics game. They wanted Gemany to do the firstmove, every time when it came to their grievances. Attacking Germany when they were invading Poland would have looked like it was Germany who was under attack. Ignore Germany's diplomatic requests regarding Poland and let them attack Poland instead. Escalate the situation and corner Gemany with a righteous war declaration. The contributing factor was that they didn't quite feel that they could yet take out Germany in a swift pass. Attacking Germany when they were annexing Sudetenland and Austria would have been even worse as these populations wanted to be part of Germany anyway.

The silent factor in this whole enterprise was the Soviet Union. A new emerging superpower was building in the East and not only becoming an industrial and economic powerhouse, but a military force surpassing Germany and the West. Germany was surrounded by two superpowers, yet had absolutely 0 support or acknowledgement from the West and instead are being provoked and then handed a war declaration. Leaving Germany militarly defenseless, like suggested in the Versailles treaty was completely absurd and would leave Central Europe vulnerable to a country that had every means and reason to take advantage of the power vacuum. The Soviet Union was also dangerous because they would in return be much safer from an attack, because of the thousands of kilometers of wasteland that seperated their industrial centers from the borders. Germany was at a huge geographical disadvantage with a new player emerging in the World, and the Brits took full advantage of that.