I think most people do not exercise because it's truly not a very rational thing to do in most contexts. Exercise in the modern world, when not part of your core occupation, is a bit of a luxury - it means you have the time, the energy, the money, and the health to do it. Many people don't. People who have more balanced lives tend to exercise more and I think the causality is in the opposite direction than most people think.
You can't "make time". You can only prioritize something else lower and replace it with exercise. This doesn't mean you ever had time. This is literally what not having enough time means. Most people don't have a nicely allotted slot for exercise in their lives. Taking it up may mean losing something rather substantial, even if it doesn't "seem" substantial. If we wanted people to exercise more, the first thing we would do is absolutely give them more time.
When I was in college, I played lacrosse. It would seem that this was very important but it was not. It resulted in falling grades for me and heightened depression due to falling grades. I dropped lacrosse because my grades were far more important. The reported mental benefits of exercise, however significant they may be, will not override the benefits of spending a lot of extra time on studying and then resting from studying. In the long run, improved grades facilitated me getting to a position where I could then afford the luxury of exercising. No amount of exercise could have done the same. Exercise is always, always secondary to being able to make a proper living and staying sane (unless exercise IS your way of staying sane, but it is not for many), and those things may very well eat up all your time. Before exercise can improve things, there needs to be something to improve.
I think this is a choice people make subconsciously, repeatedly. Exercise sounds like this thing you should always do because people perceive it as not having a cost, but it absolutely has a cost, and it's not very cost effective compared to many, many things. The people you see who are doing really badly in the physical department are probably hurting on time even more than you are and the solution is not for them to start exercising but for them to have a more balanced schedule so it's not so terribly difficult to fit exercise in it.
OK, there was my monologue. Now I'll move on to how and why I exercise.
I mostly exercise for a somewhat medical reason: I have a very fast resting heart rate if I don't do some exercise once in a while, combined with hereditary tachycardias. I would rather exercise than take questionable medication, and having a high resting heart rate is not very pleasant in general.
How I do it is pretty simple:
1. I've made peace with the fact that I'm not going to go very far in the exercise department and that I'm doing it primarily for the basic benefits. I'm OK with not being great at it, occasionally not doing it, not doing that much of it, not growing too much, etc., because it's still a lot better than not doing it at all, and it gets the job done with regards to heart rate. This took a lot of pressure off because I used to have a lot of issues over not doing anywhere near as well as everyone around me was.
2. Given 1, I stopped worrying about developing some amazing regimen and decided to just look for a lazy, efficient way to fit exercise into my schedule. I chose to join a kickboxing gym for a wide selection of reasons:
- I don't have to research or manage it. I just show up to the class and the instructor takes care of things;
- it's in a closed gym. This is important because having to worry about weather used to really mess up my running schedule, due to high heat or thunderstorms;
- it's scheduled, so I either have to go do it or not do it. I can't "do it later". Combined with it being expensive, I don't want to not go do this thing I spent a lot of money on;
- while scheduled, it's still flexible enough that I can somewhat adjust to wonkiness in my schedule, but not too much;
- it's very efficient for time spent compared to most other forms of exercise I've seen;
- it's relatively well-rounded so I'm not doing either just cardio or just weights;
- it's social so there's the added effect of performing a bit better because other people are around, the coach is watching you, etc.;
- it scales off of everything so it's trivial to branch out if I want to add more cardio, lifting, etc., to it;
- kickboxing gyms are not rare so if I move somewhere I'd probably still be able to find one.
Cons: it's expensive. I can afford it, a lot of people can't.