JW's should reject hand washing for the same reasons they reject birthdays:
1) There is not a single example of any worshiper of Jehovah washing his or her own hands in the Bible. The only examples of hand washing in the Bible are from people who did not do what was right in Jehovah's eyes.
2) Jesus is the model whose footsteps we should follow closely. Jesus didn't wash his hands, so would it really be appropriate to wash our own?
3) Ritual hand washing has its origins in false religion, and while most people no longer associate it with religious practice, Jehovah does not forget its origins and we must do all things for his glory.
4) While the Bible does not outright forbid hand washing, it doesn't put hand washing in a positive light.
They should also reject it for the same reason why they reject political involvement:
1) The Bible tells us that it is through God's kingdom alone that all disease will be done away with. All other attempts at reducing diseases stand in opposition to Jehovah's will and exist to do nothing more than turn people away from worshiping him.
2) True Christians are to be no part of the world. Could we really claim that we were upholding Christian neutrality if we engage in the same cleanliness practice as our unbelieving neighbors and relatives?
3) When Jesus was offered the chance to wash his hands, he turned it down, because he knew that cleanliness belonged to Jehovah alone.
Lastly, they should reject hand washing to prevent disease because it reinforces the LIE of evolution!
1) For any new diseases to exist would require positive mutations, which only occur extremely rarely, and quickly disappear from the populations from which they originate.
2) Fossils of ancient microorganisms show little to no variation over the course of several centuries.
3) If COVID-19 evolved into a more virulent they why do other, less virulent viruses still exist?
4) Could a virus so irreducibly complex have come about by random chance?!