The “pub” in UK tends to be a gathering place for locals. Pubs are frequented by local people who usually live within walking distance and are going to be in there several evenings a week to hang out with their friends and neighbors and chat. People tend to go to the same one, or maybe the same one or two, on a regular basis, sometimes for years. A lot of them also serve food so you can spend a couple hours there drinking and eating. In US, the closest equivalent is the “old man’s bar” where retirees will be sitting in there passing the time, or maybe some of the off-the-beaten-path locals-only establishments where you kinda have to know someone and know where the bar is to even find it. Outsiders or anybody who’s disruptive of the atmosphere are generally not welcome.
The “bar” in USA is usually designed to draw a crowd of different people, many of whom will drive there for a specific reason (craft beer, a special event night, a band playing etc.) and be more interested in meeting and mingling with new people and/or getting drunk and/or dancing or grooving to the band, than with sitting around chatting to a friend for 3 hours over a pint. In USA, people who want to have long conversations will more likely go to a coffee house or a diner. Bars are often loud so you can’t really talk in there anyway. People will stay for a while, drink, dance whatever then go someplace else. The food selection is usually lacking unless it’s a “gastropub” in which case it’s more like a restaurant, it’s probably expensive, and you’re kind of expected to eat and get out and not take up a table for 3 hours discussing philosophy. It’s USA, everybody is in a rush, there isn’t much loyalty to a particular bar. If a better one opens up a mile away, everybody will drive there. It’s also not part of the fabric of daily life for most US people nowadays. If you’re sitting around a bar several nights out of the week and you’re not retired or a disabled veteran or something, people tend to think there is something wrong with you or you have an alcohol problem. In the UK it would be regarded as more normal.