Because it's not taught is schools anymore. Many parents EITHER... don't have the time to teach their kids, or they CBA to teach them, or they don't want to fork out the money, or can't AFFORD to fork out the money.
Even before the issues with the economy right now/covid/brexit etc, one in four children couldn't swim. (25%.) I think it's close to 4 in ten now that can't swim! (40%.)
Some parents really genuinely don't have the time or money for lessons, but to be honest, some parents just can't be bothered.
The children who CAN swim will typically have parents who spend time with them reading, watching films, crafting, taking them for walks, to museums, to nature reserves etc etc.
I couldn't imagine not being able to swim, and can't remember a time when I couldn't. Most people my age/generation learned to swim. It's shocking how many younger people and children can't these days. It's such an amazing life skill. Yes, it really is.