It's because in the US medicine is a postgraduate degree. This is not an uncommon model for many countries.
You need to do an undergraduate degree first, during which you will complete the premedical curriculum. Premed isn't a degree/major, it's just a set of courses you take while completing your first degree. You can major in whatever you like - many premed students pick a science major since they have to take a number of science classes as part of the premed curriculum anyway.
The premed classes are standardly 2 semesters of biology, 2 semesters of physics, 2 semesters general chemistry, 2 semesters organic chemistry (or 1 semester organic chemistry plus 1 semester biochemistry), 2 semesters of calculus (or 1 semesters calculus plus 1 semesters of statistics), usually completing whatever your colleges writing requirements are, and sometimes it's recommended to do 1 semester of psychology. If biochemistry isn't completed as part of the above usually students take 1 semester of that in addition to it anyway. So it's just "basic" science classes for the most part, it's not clinical content or (applied) medical sciences which is the material covered in the medical degree.
You also need to do the usual work experience, shadowing, preparing for the MCAT, and decorating your CV otherwise as well as doing well in your academics