Good reasons why teachers don't correct all your mistakes

1. Class time - If the class is 60 minutes long and the teacher spends 30 minutes correcting student mistakes, that only leaves 30 minutes for speaking, reading, listening, writing, checking homework, setting homework, explaining the new language of the day etc.
2. Slip ups - Some of the mistakes you make are just because you are tired, thinking about something else, concentrating on different language etc. If so, you already know that language is wrong and the teacher pointing that out to you is not very useful.
3. Relevance - The mistake you have just made might not be connected to the language in your course, might not be the language you need to reach the next level, or might not be something the other students need to hear about.
4. Concentration/ distractions - If the teacher corrects you on many different unrelated points of grammar, you will not be able to concentrate fully on the most important ones or on the language point of the day.
5. Fluency - If the teacher corrects you every time you make a mistake you will always be thinking about mistakes and that will slow down your speaking. Speaking very slowly and correcting yourself all the time will stop you reaching the next level and will make it hard for people to talk to you without getting bored and impatient. It will also slow down your reading and writing speeds, and make it hard for you to listen to people speaking at normal speed.
6. Expanding your language - If your teacher corrects every mistake, that will also probably make you only use easy language so that you know that it is right. To be ready to go up to the next level, however, you need to be ambitious in your use of language and try to use each new word or new grammar item any time you think it might be possible.
7. Natural learning style - Many people do not realise that children learn their first language (mother tongue) without much correction. One of the stages they naturally go through is using new grammar they have just learnt too often (I passed, I buyed x, I seed x) for a few weeks or months until the language has been properly learnt. Most teachers and researchers believe that people learning a second language need to go through the same stage with new grammar, and that being corrected a lot at that time does not help students to speak more accurately and may even confuse them more and slow down their progress.
8. Saving mistakes for later - Your teacher might be saving your mistakes on paper or in their head so that they can do the error correction when you can properly concentrate on it and/ or so that they can choose the most important mistakes to concentrate on in this lesson or future lessons.
9. Introducing new language instead - If your teacher has to spend lots of time correcting you on a basic grammar point and so can't move on quickly to the next grammar point, that might hold you back from reaching the next level. Most teachers and researchers believe that reaching the point where you don't make mistakes on one grammar point takes time however you study and however often you are corrected, so it is best to move onto another point for a while and then go back and revise rather than keep repeating the same correction until you never make mistakes.
10. Confidence boosting - One of the most important things you need to speak fluently and keep your motivation to study is confidence in your ability to communicate. If the teacher is always interrupting you and correcting you, it can be easy to become nervous about speaking.
11. Negative reactions - Even though you know you need correction, it is possible that when your teacher does make a correction you usually look disappointed or even angry. If so, your teacher might be nervous about correcting and not believe that you really want more correction. If you are open to correction and always remember that your teacher is not saying your English is bad, they might correct you more often.