If you can think of a better mechanism than hitpoints for modeling combat damage, then go ahead.
Meanwhile yes hitpoints arent realistic but neither is your own idea of "realism".
Sure, in reality a single hit MAY take you out, but it may also not.
And there are countless strategies in reallife how to handle combat that show that a single hit has no guarantee to take you out.
For example the whole "stopping power" discussion for firearms, where nobody has yet come up with a valid definition of what "stopping power" really means, except that those guns with high stopping power have a better chance to take out an opponent in one hit than those with less stopping power. Even the smallest gun can still kill on one hit though.
A fully realistic simulation of combat would have to model the whole physical body and take all the many factors into account. Also healing would take weeks, sometimes months. I will have to argue thats not practical and nobody would like it over the yes massively oversimplified hitpoint bar.