For most of the things you actually do in practice as a scientist in that field you generally need to be trained by someone that knows the particular area of bi

For most of the things you actually do in practice as a scientist in that field you generally need to be trained by someone that knows the particular area of biology and the specific methods you're using. It's not just about the general background knowledge, which is certainly important, but on top of that you need specific experience for each specialized area you work in. Biology is a vast field, and there is simply an enormous amount to learn. And you also need to understand the methods and techniques used to perform experiments, which can get pretty deep into physics at times, and requires a lot of very specialized knowledge for many of the more complex methods. Statistics is also important for many types of experiments. It's really not easy and fast to gain the necessary knowledge, people in the field generally have a bachelor/master and a PhD, and that is the start of your scientific career.