Language is a complex symbolic and semantic structure. Any system that tries to understand or imitate it needs to model relationships between a word and any other word in context. This can get complicated very quickly.
Earlier models could only look at a sequence of words step by step. They struggled with long term dependencies. An example of how complex language can be was recently illustrated in this excellent youtube video: Noam Chomsky on language
Consider the following sentences:
Sentences 1, 2 and 3 make sense when you hear it. But sentence 4 is just an absurd statement which doesn’t make any sense. Though the only difference is the substitution of the word “ready” with “hard” - though both are adjectives