Build vs buy is a huge, complicated topic and it's more than just a technical conversation. A whole dev team is likely way more expensive than a SaaS for almost anything. You may think you can build it as a one-time cost and then maintenance will be cheap, but that's a trap. Every bug, every feature request is now on your queue and has to get prioritized against features that are actually core to your business. If you're the manager, how are you supposed to manage the risk of losing the team members who remember how it was built? You may think that hashing passwords is a solved problem so this isn't hard and that's also a trap. Building a fully-baked authentication platform is way more than just hashing passwords. You need all the UX flows for passwords, Oauth, email/sms recovery. Integration to your CRM or CDP. Obviously it's all solvable and possible, but why would you?

You can put the same logic behind loads of other enterprise software platforms. Building something like a CRM or Email management platform probably seems pretty easy, but if you look at what the SaaS products offer these days it's a mountain of features out of the box for a simple price tag. It's far, far less risky to buy than build.