system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 - Why can't we draw what our brain tells us to?
gpt
Your mind and thoughts aren't very good at being precise. They don't need to be. Comprehension of ideas and recognizing patterns for example are bigger priorities and the focus there is abstraction over precision. 
You can apply this to both the main processes involved in drawing something that you have in mind: 

The actual act of visualizing what you want to draw doesn't have a lot of detail to it. Even when you think it is, your brain is only holding a few details in mind and you are perfectly content with it. 

The act of physically drawing a detail that you can visualize is also difficult. It takes a lot of practice to get good at art in general. Imagine you literally had a picture in front of you and you wanted to copy it in your own sketch. You don't have to use your imagination or memory any more, and yet someone untrained might not be able to do a very good job.