Because you can’t learn everything from work experience.
Medicine is not for everyone.
Every person experiences medicine in a different way. Your parents/extended family/family friends may think that medicine is the best career in the world (or even the only career in the world), but you aren't them, and you should NEVER apply to medicine just because someone told you to.
You won’t actually spend that much time with your patients.
When working in a hospital, your ward round consists of spending around five minutes per patient, and the majority of the time you'll be transcribing the conversation your senior is having with the patient. Only certain specialities, like family medicine or care of older people, involve getting to know your patients.
In your first years, you’ll spend more time doing admin than anything else.
You'll graduate from medical school with so much excitement and determination to help people. But when you start as a junior doctor most of your time is spent doing paperwork, running around the hospital delivering paperwork, and requesting scans for patients you may have never even met.
Medicine is more than just liking science and wanting to help people.
Medicine is about learning something new every day and developing your people skills to deal with angry family members. The pure science you learn in your first two years is rarely used on a clinical basis.