Even if we’d understand precisely how every neuron in our brains work at a molecular level there is no reason to believe we’d understand how we think.
We can’t simply reduce one layer into another and expect understanding.
We simply dont know how to make a model that works like you seem to want. Sure, we could start over from scratch but there’s an incredibly strong incentive to build on the capability breakthroughs achieved in the last 10 years instead of starting over from scratch with the constraint that we must perfectly understand everything that’s happening.
I don’t think we can. Maybe we find some mathematics that let us build the model from first-principle parameters. But I don’t think we have something like that yet, at least nothing that comes close to training on actual data. (Given biology never figured this out, I suspect we’ll find a proof for why this can’t be done rather than a method.)