I exercise because our bodies weren't really designed to live more than a few decades, and they start breaking down pretty early if we don't treat them with respect, which includes regular exercise of the mind as well as the body, along with eating healthy, real food.
All living organisms are programmed to survive, and humans, for better or worse, have been gifted and cursed with the awareness of our own mortality. Now that modern society and the sciences of medicine and food production have given us the ability to live much longer lives, we have to put in the physical effort to take care of our bodies to make those extra decades worth living, especially because we have become so sedentary.
At age 73, I hope to live at least another couple of decades, and I intend to be active and vital as I approach my '90s because otherwise, what's the point? I have always sought out exercise classes and even worked with a personal trainer at a gym after turning 60 until 2020 screwed everything up - but I've figured out different ways to keep up the fitness and I have four regularly scheduled activities every week just for exercise which pushes my limits. And you know, most of the time I don't want to do it, but I always feel better afterwards. Always.
I live in a condo complex right now with a number of older neighbors, and I've seen the obvious difference in vitality and quality of life between those who exercise and those who don't.
To me, it's a no-brainer. Mind and body, use it or lose it.