What you suggest is already the case. The book and exercises are "here" for you to do, nobody to stop you. It's called the internet. The calculus police doesn't

What you suggest is already the case. The book and exercises are "here" for you to do, nobody to stop you. It's called the internet. The calculus police doesn't come get you if you're reading a calc book in 10th grade. It's just that you don't have any way of getting instruction or school credit - so you are very unlikely to be successful, and not very likely to have your university credit you as having mastered the material without taking a college class.