Because no one in their right mind suspected what Germany would be capable of.
At the time Piłsudski proposed a pre-emptive attack on Germany surely it didn't matter one bit what Hitler and Germnay did or didn't do after 1939 as it hadn't happened when Piłsudski proposed a pre-emptive strike!
The fact that Piłsudski contemplated such a radical action meant that he must had have good reason to think this was a sensible course of action to be taken by Poland. Why wait for the British or French, or even more importantly why even ask them—given the post below?
the English (David Lloyd George, the English prime minister, who decided to lift the Royal Navy’s blockade of Russia just as the Civil War seemed to be turning against the Reds) the Soviets were granted new life.
The question is why did Poland not act alone given Polish army had seen off the Red Army? Was it because Poland hadn't actually got the financial reserves to pay for a unilateral war against Germany so needed support from others?
What was Piłsudski's plan for Germany if Poland, Briatin and France had of beaten Hitler's forces? Was he intending to carve up Germany between the three partners or just pull out and leave German to its own destiny again?