Why doesn't water wash off the coronavirus alone?
Because your hands are covered in a thick crust of dirt, dust and fats, in which the virus can hide. 

But anyway still nobody has been able to show that this virus is regularily (or even rarely) transmitted via smear infection. (If someone knows about any evidence to the contrary, please come forward. And yes, some other diseases, especially bacterial infections, do even mainly take this route. That´s how you usually get the "shits".)

It can be detected on surfaces after hours via PCR, however PCR is as sensitive to broken down pieces of a virus particle as it is to the fresh, contagious specimen. Obviously the RNA pieces also of a destroyed virus would not evaporate, so the result of these investigations proves nothing but the extremely obvious. Newspapers are spreading it as a piece of clickbait and tinder, causing people to buy lots of "sanitizers".

Anyway, use soap, leave the disinfectants to the health care people who absolutely, direly need them, and be done.