Comment by Dowwie

A vital enabler of these firms is leverage through margin financing. There wouldn't be quant trading firms without leverage, extended by a prime broker. Larger firms deploy fairly interesting, complex arrangements across multiple prime brokers. You don't want a pb to know your entire trading strategy. The PB, on the other hand, wants self-funding accounts, featuring hard-to-borrows and other types of valuable securities as collateral that can be rehypothecated and used by its securities lending businesses. Finding a viable leverage policy across multiple prime brokers is a nice, juicy challenge for the problem solvers.