For the same reason people use banks and brokerages -- it's easy. There are also significant risks to managing your own crypto: managing wallet security is not

For the same reason people use banks and brokerages -- it's easy. There are also significant risks to managing your own crypto: managing wallet security is not trivial, people have lost money because they lost their passwords etc. There is a staggering amount of scams to avoid. The level of complexity and risk inherent in self-managing crypto is just too much to expect from the general public.