The fact that bit testing also needs one instruction does not mean that it is equally efficient.

On x86-64, there are 2 ways to test the value of a bit. If you use the bit testing instruction (BT), that instruction is both longer and slower than testing if a register or memory value is null.

If you use the test-under-mask instruction (TEST), this is fast, but the instruction is significantly longer (by including an immediate constant for the mask). Longer instructions can also cause lower speeds, when various bottlenecks are encountered, e.g. the maximum number of bytes fetched per clock cycle or the capacity of the instruction cache or of the micro-operation cache.

Moreover, testing whether a value is null frequently requires zero instructions, not one instruction, because if the value is the result of computing some expression then the flags register already stores if the value is null or not (and its sign).

On ARM Aarch64, the instruction that tests a bit in a register has a much smaller jumping range than the one that tests whether the whole register is null, so testing a bit in a register may require the insertion of an extra jump instruction to reach the target where execution should continue.

Once upon a time testing whether all bits of a number are zero was slower than checking a single sign bit. Even when MIPS was originally designed, Hennessy and his team had some trouble with making BEQZ/BNEZ fast enough for their intended pipeline.