Why not the piano as a reference for bass

I think that the "attack" has a lot to do with it.  Whereas a bass is "plucked" by hand or played with a pick, a piano is struck with a felt-covered hammer and this presents a completely different "leading edge."

Steinway, Baldwin, Yamaha,  Grand, Spinet.....they all sound different

Double bass can go as low as 31 Hz, but only on five-string instruments or when equipped with a low C extension. The lowest note on a standard four-string double bass is typically an E1, which vibrates at approximately 41 Hz. However, some five-string basses have a low B string (tuned to B0), which is around 31 Hz. Similarly, some four-string basses have a low C extension that can reach down to 33 Hz. So double bass is a good instrument as a reference for low bass.

On  the other hand, yes piano can go as low as 27.5 Hz. However, modern piano started to take its present form much later than Beethoven or Mozart time periods. While the specific frequency of the lowest note on Beethoven's piano is unknown, the range was limited compared to modern pianos. The lowest note was typically F1 (around 43.65 Hz) or C1 (around 32.70 Hz), depending on the instrument. So typically one may not hear the lowest note of the modern piano when listening to Beethoven or Mozart's works. And I am not 100% sure how many times one will hear the lowest note in jazz piano either, but I could be wrong.

Hence, in my opinion the double bass is a more reliable reference of the bass than piano considering the how often one can hear these lower notes. One can clearly hear the purring of the double bass in jazz ensembles most of the time right in the middle of the sound stage. It would be interesting to hear from those who know how often one hear the lowest note of the modern piano (27.5 Hz) in classical and jazz music. 

The difficulty in using piano as a reference for bass response is that very few pieces stay down in the low bass for long enough. And the bass part is usually complementary to what's being played in the right hand so cognitively it's difficult to just listen to the bass part by itself. And if it' a grand piano, it usually has a lot of reverberation so the ADSR envelope of the notes is very different to an acoustic or electric bass. But of course, piano is still useful for gauging the bass response of system at frequencies that a string bass does not reach. Synthesisers are arguably even more useful. 

Okay so I didn't  clearly articulate my opinion of the extreme low bass on the piano..YES I know it is a stringed instrument but again I stand by my take on it's lack of texture compared to a plucked or bowed bass..
 I believe @toddalin reply explains exactly why I hear it as one note thump thump instead of the resonating,detailed and expressive bass I hear from say a cello or stand up double bass..