(1) An engineer getting paid 120k doesn't "cost" 120k, probably >150k with federal taxes, health insurance, benefits, and so on. Not including the cost to recruit, interview, and train said person.

(2) I don't know of many 1,000 person companies that would trust a new grad software engineer with no experience to manage critical infrastructure.

(3) You need N engineers to manage said service, because what happens when your one engineer gets sick, takes PTO, or quits for some reason? You also need a manager for said engineer(s).

(4) You now need to secure an internal service you never did before, so expect to have to hire external security consultants or re-allocate security engineers, since it's high risk.

(5) Github is FedRAMP compliant, SOC1 and SOC2 compliant and GDPR compliant. If you or your customers need any of those things, expect to hire external auditors on a recurring basis to validate your home-grown solution meets those requirements.

I hate to make these points because I'm a big believer in the scrappy startup mentality, but if you want to do things right, in the context of a large enterprise that is accountable to a lot of people, expect a project like this to cost $1MM per year minimum, and it probably won't reach parity with a cloud offering in terms of reliability, multi-region performance, proper backups, and so on. This is why Github can charge ~$200 per user (Or $200k per year for 1,000 seats) and still come away looking like a bargain.