Keeping work/private life separate is reason enough to me (also, we don't use Github - we use bitbucket - the decision was made for me!)
Ultimately, I think having work-specific accounts is preferable, and I wouldn't be surprised if a lot of IT departments also prefer this (however, this is also coming from the medical space, where controls are rather tight and security is extremely important and heavily audited)
I've worked for a handful of medical companies, including ones that handle de-anonymized PHI/patient data and/or develop medical devices, both very heavily audited and regulated even in comparison to other medical companies. Using a personal GitHub account attached to an organization on either GitHub proper or GitHub Enterprise was never an issue. Sure we had some folks create a separate account because they wanted to, but there no directive from the IT/security folks even suggesting that, and I'd consider them showing any preference toward not just attaching your personal account a big organization red flag - it means they're just defaulting to "don't do that" rather than actually taking five minutes to understand the constraints and ramifications.
Ultimately, I think having work-specific accounts is preferable, and I wouldn't be surprised if a lot of IT departments also prefer this (however, this is also coming from the medical space, where controls are rather tight and security is extremely important and heavily audited)