In the indoor pool where I swim regularly, I occasionally see people swimming wearing clothes on. Is it not weird? Why do people do it?

I have yet to see someone wearing a suit with jacket and tie, but I've seen people in the swimming pool wearing jeans (yes, also full leg length ones), T-shirt (ok, maybe pretended to be a swimsuit), shorts (maybe also pretended to be the swimsuit), and skirt (no way it can be confused for swimsuit).
Maybe they think they look not very fit and prefer not to show off their body?

I see an argument about hygiene, but other than that I am not really against it. Still, I find it very weird to say the least.