Since the shifter is also used for bit tests, the 'most things are a 1-bit shift' might not be the case. Perhaps they did the analysis and it made sense.

There are separate opcodes for shift/rotate by 1, by CL, or by an immediate operand. Those are decoded to separate microcode entry points, so they could have at least optimized the "RCL/RCR x,1" case.

And the microcode for bit test has to be different anyway.