Because nobody had the power to rationalise it. After 1066, the upper classes turned their backs on the germanic language, in favour of something similar to normandy-french, and the peasants simplified/dumbed-down the grammar of the germanic one.
English is teeming with synonyms, homonyms, homographs, homophones, colloquialisms and irregularities. It has spelling contests for *native* speakers.
If you think English is easy, learn Esperanto