The methods of research and the whole paradigm of science has now become such, that you typically do a phd with a guide in a specified area, which is more or less your first restriction of doing science. Then, once you finish that, you get post-doc in same or allied area. i.e. if you are in say quantum physics, you can’t easily switch to stat mechanics. and so on. so now you are done with post doc, the agencies that hire you are going to rely on that speciality. the funding will be related to that area or topic only. and so one continues until the end of research career.
where is the free thought and method and freedom to explore of a scientist!?
specialisation and logical bifurcation has led to such lack of freedom. The method of science itself doesn’t generate funds to pursue science, so one has to rely on funding agencies and do the science that they wish to do.