I'm not sure speaking English is as easy as you think. I'm a native English speaker and have spoken the language for 33 years and I still quite easily butcher the language. A lot of it probably has to do with many words have two meanings and sometimes depending where you are different spelling and pronunciations. Every year hundreds of new words are added to the English language it's constantly evolving. Those are just a couple of ideas that come to mind. Also you might like this I found it on a google search. 21 Reasons Why The English Language Is Hard To Learn: 1) The bandage was wound around the wound. 2) The farm was used to produce produce. 3) The dump was so full that it had to refuse more refuse. 4) We must polish the Polish furniture. 5) He could lead if he would get the lead out. 6) The soldier decided to desert his dessert in the desert. 7) Since there is no time like the present, he thought it was time to present the present. 8) A bass was painted on the head of the bass drum. 9) When shot at, the dove dove into the bushes. 10) I did not object to the object. 11) The insurance was invalid for the invalid. 12) There was a row among the oarsmen about how to row. 13) They were too close to the door to close it. 14) The buck does funny things when the does are present. 15) A seamstress and a sewer fell down into a sewer line. 16) To help with planting, the farmer taught his sow to sow. 17) The wind was too strong to wind the sail. 18) After a number of injections my jaw got number. 19) Upon seeing the tear in the painting I shed a tear. 20) I had to subject the subject to a series of tests. 21) How can I intimate this to my most intimate friend?

there are lots of reasons behind this cause. but I think the most important reason is that people like to speak rather than to write. And It's boring to always follow the rules. In speaking, you try to communicate, so when your partner understands you,even if you make ungrammatical sentences, you feel happy and satisfied. In grammar, you try to follow the rules which force you to be accurate.otherwise you will feel bored and unwilling. that's why speaking is easier than writing, using grammar.

Speaking English badly is easy, speaking it properly is extremely difficult. With the exception of native speakers, almost everyone fails.