Hi Mint, You may have heard that "We may not write the way we speak; we may not speak the way we write". Unless we are speaking in a very formal context with a prepared script, we usually have to think spontaneously as we speak, and it's very common for us to retrograde and add on to what we just said. Moreover, there are many interjections that we use in speech. Interjections are sounds like "uh huh", "oh" that have no specific definition but serves the purposes of expressing intention, interest, mood, agreement and so on. Sometimes, we may not speak in complete sentence structures but a native speaker is still likely to piece together everything and understand the conversation based on contextual and lexical clues. For example, a police officer can interrogate someone by asking, "That right?" instead of "Is that correct?" If we throw in regional accents, slangs and colloquialism, you may start to realise just how many elements there are in speech. Of course, there are formal and informal writing as well. Readers can slow down and re-read a sentence many times if need be, but a conversation is carried out in real time, thus the "challenge". Reading and speaking activities in English lessons are usually carried out in a controlled and graded manner, and may revolve narrowly around a theme (e.g my holidays, crime, transportation) but a conversation may branch off into different topics. These are just some of my opinions; I'm sure there are some members who can offer a more thorough explanation than me. All the best!

Assuming you are asking for an answer to this question: It's probably because conversations in lessons are between a student and a language teacher whose goal is to help you improve your speaking skills. Real life conversations are between you and any number of different people in various situations with different motivations and interests. These people aren't listening to catch mistakes so they can correct you like teachers will. Also, your relationship to people in real life is probably not student/teacher so that will change the nature of a conversation as well.