Pre-existing industries and investments, which is the same reason why the British also out-produced the Germans in terms of the number of ships.

Economies are not made equal. While Germany had considerable steel production for artillery it had a very poor civilian aircraft and a stagnant shipping industry before the war. Britain by contrast made aircraft production a priority when they began re-arming in response to German militarism by 1936.

On the other hand, this meant that British tank and ground forces development was always considered to be a secondary objective - to the point that the British military did not feel confident it could fight a proper ground war until 1939 and this was in fact one of the major reasons for the Munich Agreement - while the British were prepared for a naval or air war by this point, they had no real army yet. They were instead dependent on the French, who had been mobilizing its army, but the French ultimately chose to abandon their Czech allies.

Capturing France in 1940 was not enough to overturn these pre-war investments and economic realities. Indeed, French aircraft production wasn't exactly great either, so capturing French industries didn't help much in this regard.