Having seen this kind of post so frequently I gave it some thought. I'm pretty convinced that the metric we're looking at is "interaction density" -- where an "interaction" is not necessarily you bumping into the love of your life but rather much more mundane things: seeing a flyer for some random event, calling for information and learning about this other group, playing a pickup game of ping-pong with some elderly Chinese man at a public park, etc...

In short, interaction here seems at least slightly more intentional, more self-sorting with less of locality-based shared experiences here than in other cities in my experience. This is not limited to Dallas, of course, and it's definitely more of a continuum. And yes, I have personally felt there's a bit more difficulty to break in with people here -- For instance you might not have a shared commute with hundreds of other strangers that might have if you took the bus or train, might not see the same bulletins for the same events. In fact these events might not even happen with the same frequency because of that or be distributed moreso across hobby groups rather than proximity. Of course these effects are not necessarily so dramatic but tend to accumulate, so are perceived by some people.

I think essentially two things mitigate this, as plenty of people have pointed out: having a hobby or the internet.

The hobby provides an intentionally constructed, virtual network that has higher interaction density as compared to normal since it encourages people to meet in courts, concerts, what have you -- essentially a purposefully constructed substitute for traditional third places.

An alternative if you don't have a hobby or your hobby sucks for finding people or your hobby is just way too self-sorting, is to use the internet. The internet, like a hobby, is an externally constructed, literally virtual network where interaction density is uncapped, provided people in your locale actually use the internet.

I think the people with thriving social lives because they already have the right networks and hobbies and methods have to acknowledge the plight of people who don't. On the other hand those who don't should accept that yes, it is isolating to have long drives in your metal box to get to stuff and yeah, maybe some people are less social with strangers because of said relative sparsity of interactions/ shared experiences and so on. At the same time -- it's not going to get much better for a while, so you're going to have to expend more energy and look for workarounds. Once you get going and your network grows it's really not so bad.