Anyone who has ever transcribed an interview, conversation, or unrehearsed speech of any sort will be very aware of how disfluent this form of language is. We start and stop and stall, repeat ourselves, insert filler phrases and sounds, search for forgotten words, abandon trains of thought, deal with interruptions and distractions, follow tangents, and double back.
It’s a far cry from writing, which gives us the luxury of time to prepare and arrange the translation of our thoughts. Grammar is therefore normally much tighter in writing than in speech. And because we learn about language through writing, significantly later than we develop speech, we cannot help but find speech wanting when we (unfairly) compare the two.