"No developer in their right mind wants a developer portal"
This was a quote I heard from a platform PM. And he was right! It reminds me of the similar quote: "People don't want to buy a quarter-inch drill. They want a quarter-inch hole!"—Theodore Levitt
It might be a tough pill to swallow but:
No one wants your platform.
No one wants your tool.
What developers actually want is what your product enables them to do. Shipping faster, feeling more autonomous, mastering their craft.
Developers don't want a developer portal. Developers want to be better developers.
A little while ago I worked on an integration where I had to implement API endpoints with specific functionality defined by the integration partner, and I found the test functionality they built into their developer portal very helpful. Rather than having to write tests based on their documentation to verify I was implementing the endpoints correctly, I was able to test against the real code on the other side of the integration.
Honestly for companies that require you to log in before accessing API docs, I imagine the decision is being made by non-developers. I suspect the alternative would be getting a PDF emailed to you after having to ask a real human for it.