You can meet people here and there by going to Meetups and whatnot, but it's hard to go beyond casual acquaintance. You can't ask the same person to hang out too often, or you start seeming desperate and they will value your time and your friendship less.

And a bunch of one-to-one connection don't work, you need to build an interconnected network. But if you already had a nice group of people to connect newcomers to, you wouldn't have the problem in the first place. If you're lucky, you'll make one acquaintance that will pull you into their own social network, but that's not easy. If it's a network of people who have all known each other for a while, you'll always be the outsider.

If you want to try to put a network together yourself, you need to organize activities where strangers (your individual acquaintances) can meet and start forming connections. But it's pretty hard to get several people together at once. If you want to go to some event, you need to find something that everyone will like. And even then, it needs to be repeatable. You need to get your friends to meet several times, so one-off events, like a festival, won't work.

The best option, in my experience, is a house party. Everyone likes that, and you can have it however many times you like... provided you have a house where you can host people. In some areas, such as S-fucking-F, this is hard and/or very expensive. And of course, you need to be the sort of person who can act as the hub of a social network. If you're introverted, you probably don't have the right skills.

If you can't be the hub yourself, your best bet is to befriend someone who can be the hub, so they'll build a social network that includes you. But if that person has been around a while, you're back to the outsider problem. They already have a network, and it doesn't include you. If you find someone who has hub-skills and is new in town, you may have struck gold. Of course, their social network will start to fall apart once they leave town.

We did this by moving to another country where there was limited potential to really integrate into the local community (Thailand).

Because expats existed in their own little community and everyone walked off the plane knowing no one all the new comers are extremely open to forming new friendships. At to this that moving to another country is a challenging experience which you are going through together and there is a very high potential for making a whole new group of friends.

The only similar equivalent I can imagine would be something like joining the military. Everyone shows up knowing no one then they all undergo a series of challenging experiences together. It is not surprising that some military people seem to form lifelong bonds with their peers.

Meanwhile if you stay wherever you usually live, an environment relatively lacking in hardship where everyone else has at least a semblance of an existing social network, making a whole new social network is extremely tough.

If I did this and the relationship ends 20 years from now I could find myself being an old guy without any friends and then it's harder (but still possible) to find new friends. (because you get rusty when you didn't do this kind of thing for decades)

I can definitely see why some people then choose to sit in front of a TV all day, because with your friends probably also a lot of the activities and hobbies you once enjoyed are gone.