why didn't britain and france stop hitler before germany became a superpower?

They had been traumatized by WW1, particularly France.

One could go on about this for ages - and pages. But the very short version is: Because they did not take him seriously. - Nationalism and antisemitism were not uncommon in Europe at that time and not only people in Germany were "blind on the right eye" (as in right-wing people would get away with a scratch while left-wing people (who probably did the exact same thing) would be severely punished).

The British an French government, as well as the German government (until Hitler took over), thought it would be enough to give him just enough power to make him shut up and then "push Hitler in the corner until he squeezes" (Franz von Papen) to keep him from gaining influence. - This did, obviously, not work. He was clever enough (or had people who were clever enough) to make things look legit (seemingly "legit" Enabling Act). - After he had Germany (the government & the people) under his control he did the same thing internationally; He used Wilsons idea of the self-determination of nations to make the annexation of Austria (well that one was the most legit one) and later of the "Sudetenland" seem legit (like; There live German people? - Why aren't they allowe to belong to Germany? - We have to "bring them back home"). - And tried it for the "Rest-Tschechei" as well (which did not work as easily because there were almost no Germans living there). - Around that time the British and French government kind of started to wake up - but up until then they thought they were preventing a war (by giving Hitler what he wanted).

You might have a second look at "the Munich Agreement".