Why is the withdrawal method not recommended to prevent pregnancy?

See below. The penis secretes fluid that has small amounts of sperm in it from the moment it becomes erect...Not just with ejaculation.
High failure rate. Of 100 typical couples relying on withdrawal to prevent pregnancy, after 1 year, 22 will have gotten pregnant. With perfect use, the failure rate is closer to 4/100. Withdrawal is also difficult to practice consistently and perfectly.