I'm not an expert, but from personal experience I would hazard it's mostly due to not having time to cook. And perhaps more than not having time, it's not having the energy.

After work, I don't want to cook. I might be physically exhausted from my job, or cognitively drained from dealing with bullshit. Some people work several jobs, some people have hours-long commutes. Then there's obligations like childcare, family, or maintaining your social life. So some people literally don't have enough hours in the day, and for others although there are technically enough hours, the time must be spent on recharging your batteries.

Some people recommend batch cooking and meal prep to counter this, but I personally find it untenable. It's challenging to find the will to make a single meal, making one that's five times larger is an ordeal. Add to it that I don't really want to eat the same thing every day, and it's a tough nut to crack.

There are almost certainly people who don't know how to cook. Public education has been gutted in the US, so to learn cooking you probably have to rely on either your parents teaching you, or having the motivation to learn it yourself. If your parents are also struggling to find the time to cook, they probably don't have the time to teach you either. Self-taught learning can be a crapshoot for most people: it can be too open-ended to know where to start, or discouraging when you fall into a newbie pitfall, or you focus on the wrong things.

That's not to say it can't be done. Self-teaching is a valuable skill that will take you far, but perhaps ironically, you have to learn how to do it yourself. Partly due to its nature, and also see above where public education has been gutted.

As for it being cheaper: maybe, but not likely. Some meals it might become cheaper to purchase that way due to economies of scale in processing, but it's almost certainly going to be cheaper to buy the raw ingredients yourself. Exceptions might apply for stale-dated food: something that the grocer was going to discard but gave it a second life by processing means they might be able to afford to lower the price. If you're not picky about what you're eating, a ready-made meal that's about to expire will probably be discounted. I still don't think it would be cheaper than buying ingredients, but it's definitely cheaper than a restaurant.

One thing not mentioned is access to equipment. I don't have a rotisserie or a deep fryer. Some people might not have a full size oven, or are forced to do most of their cooking on a hot plate. Still others might not have access to a kitchen at all.