1. I would argue they're not a net positive. Making markets more efficient is fine but markets are already extremely efficient. The biggest social good can be achieved by working on issues that are classical market failures and full of externalities. Climate change, healthcare, basic science, education and so on. The man-hours invested in these prop trading platforms compared to the benefit to society is maybe positive under the microscope, but negative when one considers opportunity cost.
2. They're freakishly proprietary. They're corporate black boxes full of 200 IQ people who could be solving and distributing knowledge, but they're basically locking them in the basement to make money. Put any of those researchers into a public university and over the course of their career, they could educate thousands of fine researchers.