It's all about concentration. Yes, water has a pH of 7, but the moment we add anything even remotely acidic or alkaline into the mix, the pH of the entire mixture will shift up or down. (This is called an acid-base titration - where you add a titch of something to see how the pH moves up or down.)
Let's say you have a cup of really acidic stuff (let's say hydrochloric acid) and a tablespoon of water, you aren't going to see a huge shift in the pH of this cup of stuff. It's still around 1 - very acidic. If you add 30 ml of water to 250 ml of hydrochloric acid, you still won't see a big shift. Even if you added 250 ml of water to 250 ml of hydrochloric acid, you still won't see a pH even remotely close to 7. Add 1000 ml of water to 250 ml of hydrochloric acid, and you still have an acidic pH. There will be a point where there's only a titch of hydrochloric acid in the mix and you will still see a slightly acidic pH. You can't get back to 7.
The same goes for something really alkaline. Even something like 250 ml of lye added to 250 ml of water won't become pH 7 or neutral. You could get it down to 5 ml of lye and 250 ml of water and the lye is still strong enough to shift the pH to the alkaline side.
If you want to do a little experiment with this at home, add 5 ml of vinegar - 5% acetic acid, which works out to about 0.25 ml acetic acid - to 250 ml of water. Now drink it. You can still taste the acid in the glass! Ick! This indicates the entire mixture has become acidic, which means the pH is no longer 7! Please do not try this experiment with strong acids or alkalines - vinegar or lemon juice only - because you could die. I've warned you. And like I say to my craft group kids, if you drink it and die, it's not my fault because you've been warned. (From the strong acids and alkalines, not from the vinegar or lemon juice).
So let's extrapolate this to our bath products. If we add something with a non-neutral pH (like shampoo, conditioner, or body wash) to water, the water is no longer neutral. It will shift towards being more alkaline or more acidic. Rinsing away an acidic or alkaline thing with neutral water won't shift the entire mix to being neutral - it will make the water more or less acidic.
Once the water is acidic, it takes a whole heck of a lot to make it non-acidic. On top of everything else, the water we use in our homes isn't really neutral - because of various metals, our pipes, acid rain, carbon dioxide and so on - it's a titch on the acidic side.