If they were more efficient per watt for scientific computing, you’d hear about researchers building HPC clusters from them.

Efficiency per watt is not the same as total cost of ownership. Pis are expensive for the amount of compute you get from them in total, but the compute itself is efficient per watt. You would need at least a dozen Pis to rival the latest CPU processors in terms of total output, a dozen Pis is more expensive to buy than a single CPU.