Please tell me a few specific reason why you think that 100% accurate machine translation does not yet exist?

One of the reasons is that you need a perfectly correct input in order to get a correct output.

Another reason is that most services use only one of the MT methods and do not try to combine them and get a good result.

Does any system exist where one could enter the electronic text of Homer's Iliad in English, with no other user input or interaction, and return a grammatically correct translation in German, Chinese or Greek? If such a system existed the world would know about instantly, and all language translators would be out of work forever.