Why didn't medieval infantry strike cavarly with long pike? Why was cavarly so strong?

Since the Charles Martel's Battle of Tours (7th Century) to 14th Century, the strongest medieval military force was cavalry. But I don't understand this. As depicted in the Braveheart movie, infantry (foot soldiers) can strike horse with very long pikes easily (much longer than knight's pike). This is not a rocket science. Anybody can think of this. Then, why wasn't every infantry try the same?? Pike Square strategy was not invented until late 15th Century!! Why?