I think an important distinction should be made between dentistry and orthodontics. I believe that in many countries with public healthcare, dental coverage is pretty normal. What many governments don’t pay for is orthodontics (teeth straightening, braces, bite fixing, etc) and so most people go without it (eg memes about British people having crooked teeth).

In the USA, orthodontics is a huge industry. It’s all about having straight perfect teeth. I don’t know why it started, but the reason it’s stuck around is mostly aesthetics and inertia IMO. If everyone around you has straight teeth, you’ll feel left out if you have crooked teeth. It’s also a huge moneymaker for dentists themselves. I avoided dentists for several years because I got tired of them trying to sell me expensive aesthetic services, like whitening or special bite splints.