Why do you have to memorize a language's grammar?

I don't mean memorizing individual grammar rules (in the sense of "present tense has x, y, z conjugations). I mean, when you see something like "this word ends in this letter, this verb has this ending", why do you have to memorize it? Why can't you just kinda know it like you know things about humans in your native language? I mean, when you look at a person, you don't consciously recite the steps of how you see what emotion he's expressing, where you're looking to make that determination, you just do.<br><br>It's always been difficult for me to do that, it just seems like it's impossible to "just pick up". I could memorize a word and it's definition, and then memorize the verb and its conjugation, but I can't memorize where the verb goes in a sentence, or how to form a sentence, or how to ask questions, or all of that, it's all just too much to memorize. I feel like it's too much to memorize for any person to memorize. I can do that much, but I want to be able to just know how to say something. I don't want to have to think about how to say something. I want to be able to just say it.