You have to understand the historical cultural context to be able to understand why squash is still so popular.
If you go back to pre WW2 there was a nutritional deficiency identified in children as "the energy gap". This is where, in families that struggled to provide adequate food, the children often went without enough to give them the energy that they needed. The superfood identified to solve this problem: sugar!
Another issue identified a bit later and pushed by the health services was lack of vitamins, in particular vitamin c. A solution to this: squash. Ribena was very heavily marketed as being healthy for young children.
So that led to most children with well-meaning parents being raised on sweets and squash.
Now sugar has been identified as bad for teeth, so the manufacturers have taken the sugar out. They then advertise it as being healthy. Most people don't drink plain water and are used to squash, so they just switch to sugar-free rather than water. They still have the cultural reference that squash, and sweets, is something that you give to children, so that's what they do.
Coming from a different culture, it will look pretty strange, but it's all down to people following the best advice of the day and that becoming part of the culture. Once an idea has been ingrained like that it's hard to shake off. It's a bit like the high-carb low-fat diet advice that's been so popularised over the last few decades. They got that wrong too, but it's going to be really hard to switch is all back to a sensible diet.