Why do we say that spectral efficiency is measured in [bit/s/Hz], not just [bit]?
The units of spectral efficiency should be thought of as (bits per second) per Hertz.  It measures the ratio of the data rate to the bandwidth.  Yes, dimensional analysis shows that the dimension is equivalent to just bits but using that simplified unit for spectral efficiency does not show what the meaning of the term is. So, it is better to use (bits per second) per Hertz as the unit, or, for those whose (US) keyboards have broken shift keys and cannot type parentheses (the ( and ) come out as 9 and 0 respectively), as bits per second per Hertz or bits/second/Hertz.  (Typing / does not need the shift key).