This much I agree with. String theory people often don’t have great explanations of why they are solving the problems they are trying to solve, and even when they do they often don’t explain it clearly.
There’s a related and even worse spanner in the works of elementary particle physics: particles (or fields, strings, or whatever) are supposed to be fully quantum-mechanical entities. But the people who work on them only ever construct classical, single-universe theories. Why? Because they think that the quantum part of the theory necessarily has to be trivial.
Deutsch is just dead wrong about this.
I don’t know exactly what a “fully quantum mechanical” entity is, but I don’t think that the particles in QFT count (nor do I think strings or branes count, but that’s a more complicated discussion). Particles arise when you try to write your path integral as an asymptotic series in ħ, so the assumption that quantum effects are small is built into the whole formalism of perturbative QFT.
People who work on this stuff simply do not think that “the quantum part of the theory necessarily has to be trivial.” I thought that every theorist knows that this quasi-classical / perturbative way of studying QFT is not the whole story. Non-perturbative QFT is a huge area of study: almost everyone who calls themselves a string theorist works on some aspect of non-perturbative QFT, and “non-Lagrangian” field theories (i.e. quantum field theories with no classical limit) show up all the time.
The methods for studying non-perturbative QFT are more limited, so a lot of things about it are very mysterious. There’s not a lack of interest, though!
It is assumed that in order to discover the true quantum-dynamical equations of the world, you have to enact a certain ritual. First you have to invent a theory that you know to be false, using a traditional formalism and laws that were refuted a century ago. Then you subject this theory to a formal process known as quantization (which for these purposes includes renormalization). And that’s supposed to be your quantum theory: a classical ghost in a tacked-on quantum shell.
Since perturbative QFTs have classical limits, it’s not problematic at all that one starts with a classical field theory (there’s some extra wiggle room that comes up in one’s choice of renormalization scheme; different renormalization schemes would lead to different perturbative theories). Not every quantum theory can be obtained in this way, but I don’t think that theorists forget that.