That being said, at a certain scale or type of company centralized management, software support, and security risks mean "allowing" random people to run their own OS becomes difficult and risky. Lots of large older companies probably have proprietary software, too (thankfully more of this is becoming web based).

Yes, in theory linux is more secure and anybody who would want to use it is probably capable of taking care of themselves, but it is probably (at large companies) corporate lawyers and CISOs ruining the fun. And linux can have its own risks and dedicating a team to support (from a security perspective) them isn't economical for the what would be very small user base. Ye old big bank can't and arguably shouldn't allow it without an otherwise good reason.