We only exist because our ancestors were the right combination of badass and lazy.

In past, fitness was a requirement and a natural byproduct of life. You had to be fit to survive. The weak were killed or would probably die off. The strong, capable, and able survived. 

That was the way of nature.

Therefore, usually, people around that time were fit. Hard times made them hard work. There wasn't a fitness culture, there was a fitness lifestyle. People didn't even think about it. They just adapted it. It wasn't always pretty, and I am sure they didn't enjoy it, but it was effective.

Even in the recent past, have you heard about your grandfather's stories, where he had to cross a river to reach his school?

They would tell us stories that they would walk 15 to 20 miles a day to visit their cousin. Although it seems exaggerated, some of them were true, because of which they were able to maintain themselves without any gym or fitness club membership.
This was their life and it made them fit as a result of their day-to-day activities.

Getting fat on the other hand was a luxury. It was difficult to do. Only the rich could afford such sloths coupled with large amounts of food. So, it was attractive to be fat. It represented a high ability to gather a valuable resource, i.e., food, unarguably essential with little effort.

It represented survivability.

Therefore Fatass = Badass

Fast forward a few millennia to today, and the whole world has been turned upside down. It's never been easier to get fat than today.

We don't have to go hunt outside. We just sit down in front of our laptops in school, college, and then work, and keep on typing emails that are doing no good to our health.

We don't go without food for long periods. We have food at the snap of our fingertips, 24 hours a day. In many places, you don't even need to go out to buy groceries anymore. 

There was a time when our ancestors had to go out hunt, and several times fight with other local tribes. Their overall health required them to be active and run as fast as they can. They were also having certain kinds of traits that were specific to one in particular. For example, some people were good at running long distances, some were good at mountain climbing, and some others were having some good fighting skills. These all activities required a huge amount of energy, which their food was providing them. They were having proteins in the form of cooked meat daily, as they had to hunt for survival. They used to run, and lift weights in the form of big animals, which toned their bodies like we can't imagine.

On the other hand,

We eat more and move less.

Internet users have gone up exponentially, and there are more smartphones than people on this planet.

And thus, the fit is the new fat, and fat is the new fit.

Fitness is a luxury these days
This has broad implications for our society, our health, our appearance, our psychology, and even our evolution as a species. Attributes that were useful even one hundred years ago - just a tiny silver of our evolutionary existence - no longer are.

In short, our lives are too easy these days.

One effect of this is a natural pushback against inactivity, sloth, overeating, laziness, and sedentary lifestyles. Just as many barons and kings willingly overate and sat around to gain weight, which was attractive, now many people do the opposite. They willingly under-eat and move more than they need to.

The solutions are twofold, and both unnatural: "dieting" and the "gym".

There is nothing natural about dieting. It is just a response to a world where hypercaloric food is at our fingertips, and we need a set of rules to manage our intake. That's all. Carnivore, Keto, Vegan, IIFYM, and Paleo, all are just rules.

There is nothing natural about the gym. Squatting is not natural. Deadlifting is not natural. Nor benchpress, nor overhead press, and yes nor bicep curls.

Some are more natural than others but trust me, our caveman ancestors would laugh at us if they looked at us exercising. It would be alien to them, just as the idea of sitting at a desk for long periods would be.

This is an unnatural solution to an unnatural problem.

The more extreme the problem becomes, the more extreme the solution will also become. It is an evolutionary tug of war, as we struggle to adapt to our new environment.

That is why the gym is popular, and it will become more so in the future.