In addition to the old people thing: a lot of people who have no experience with swimming trying to swim in unsafe bodies of water they shouldn’t be in. People stop thinking rationally when they’re too hot.

Elderly and vulnerable can’t take the heat because sweating takes energy, and its so hot and humid sweating doesn’t work.

Adding to the others comments:

Given the body’s vital requirement to maintain a core temperature of approximately 37 °C (99 °F), a sustained wet-bulb temperature exceeding 35 °C (95 °F)—equivalent to a heat index of 71 °C (160 °F)— is likely to be fatal even to fit and healthy people, semi-nude in the shade and next to a fan; at this temperature human bodies switch from shedding heat to the environment, to gaining heat from it.

- wikipedia

One way is because the heart has to work extra hard to keep the body cool enough. Elderly or people with heart problems are at risk as their heart just can’t take the extra load and goes into cardiac arrest. The heat also comes with dehydration what puts even more load in the heart.

edit: and the problem is compounded again with certain medications, and elderly or people with existing medical issues are the ones using the most medication.

Young children and old people can have problems regulating their body temperature, if someone’s internal body temperature exceeds normal living range, they die.

If you want scary, look up Wet Bulb events.