Because Water heaters have to be installed as per building code. People are stupid. Families would be electrocuted or smothered depending upon whether it was electric or gas heater.
It’s that “Gov’t knows better than you how to keep you safe” mindset that conflicts with individual liberty.
This concept is what Thomas Jefferson specifically addressed when he spoke about the attitude of tyrants who believe they know better than us how we should live our lives. They force their will upon citizens through regulations, fines, penalties and other “deterrents”, “for our own good”.
Yes, these regs, building codes, etc., may indeed be for our own good. However, if citizens assume the risks and choose to exercise their liberty to install something themselves, it is their inherent right to do so.