Comments feel a little harsh. My partner does the same with dishwasher liquid, almost instinctively. I have to keep reminding them that it reduces the efficacy, and eventually will be to a point where you might as well just be using water. The other reason is that if it’s sufficiently diluted, bacteria may start growing.

I think a lot of people grew up with their parents adding water to the bottle to make them last longer, and it just became commonplace. It’s likely you can make that bottle last longer by just using less product in the first place.