That was appeasement and made sense at the time, when Communism was the biggest threat to status quo and the traumas of WWI were still pretty fresh on everyone's mind. While we know France and the UK probably could've stopped Nazi Germany's advances dead in their tracks, it came with the risk of another strenuous conflict (since the form of warfare that everyone expected was trench warfare, which was difficult enough to win the first time) when everyone was sick of fighting and afraid of the USSR (and the possibility that they would capitalise on any European conflicts to spread their influence and aid leftist groups). Just letting Nazi Germany get what it wanted and avoiding war seemed to contemporary British and French leaders to be the approach that would guarantee a future free of conflict, saving both money and lives that would otherwise be spent and destroyed senselessly.
Of course, 1939 proved them dead wrong. But we can say that with the benefit of hindsight. We know the extent of Hitler's megalomania and treachery, the Wehrmacht's tactics and the Allies' unpreparedness, and the Nazi's brutality and inhumanity. They didn't, or at least they didn't know those things as well as we do now after the fact.
So was it a poor choice in the long run and a disastrous series of decisions that ultimately resulted in the bloodiest war in all recorded history? Yes. But was it completely irrational and ASB? No, far from it.
It can't be over-stressed just how horrible the first war was. France had six million casualties in a population of 40 million - 15 % of the population. Imagine today's USA having 50 million casualties in four years. How many people would want another war after that?
Not to mention that basically all the towns along the north-east border had been destroyed. A century later farmers still find unexploded ordnance.
It was tragic that Hitler exploited the reluctance of everyone else to fight again, but that reluctance itself was quite understandable.