I'm from a less developed country and I moved to a developed country. That's pretty much what happens.

The people from the less developed country are more sociable, in the true sense of the word, because they know that in the future they will need other people. In a developed country, with enough income, a decent market economy and rule of law, strangers can do most of what you want, if you pay them or just obey the law.

In poor countries the social net is: family, extended family, neighbors, various acquaintances.

In rich countries the social net is: your own income (impersonal), the state (impersonal), the healthcare system (also impersonal). Obviously, the things I listed for poor countries also apply, but to a much lesser degree.

Personal experience, YMMV, but it's a common thing I hear from people with the same background as mine.

To some extent wealth and the comfort you can buy with it does reduce dependence on others and can help someone become isolated, since you don’t need help or favors and don’t need to engage in reciprocal social activities, since you can just pay for things. It doesn’t necessarily cause isolation, but it definitely helps someone who intentionally or unintentionally want to become socially isolated.

This. One trivial example - people used to cook their meals in my country, which meant that you sometimes found yourself lacking some basic ingredients (like flour, salt, sugar) and just borrowed them from a neighbor living on the same floor. Now, as people more people eat out or order, this has decreased significantly.

Another thing is children activity - when I was a child, all children from my area were playing together outside all day (weather and school permitting), which resulted in parents knowing each other via various everyday interactions. Now, when people can afford to pay for extracurricular activities for their children, they just chauffeur them to their various activities and there is no parent community where the children actually live.