It's usually not a question of laziness, but just path optimization. For big raid events, limited only to a few hours, Pokemon Go players will do whatever gets them to the most raids in that fixed amount of time. In places where the gyms are well placed in walkable parks, there are enough gyms that are close enough so that you can loop through the park, giving the system time to spawn new raids in the first gyms you already raided. Doing raids walking is actually preferable to driving, because in order to take down raid Pokemon, you also need a big enough raid party, and that's easier to judge with a crowd of people who agree to go in the same direction, instead of mystery players in mystery cars.

But in SO many places in the US, the gyms just aren't close enough. It's both a fault of the urban environment and the game system, but it just frequently creates situations where if you want to catch a time-limited raid Pokemon that's shiny or has high stats, driving is often the only way to play in your area.

What's funny though is that car or not, there are lots of Pokemon Go events where people don't talk to anybody. It's just a bunch of people who show up to the same spot, nod hey at each other, and then tap away on their phones in awkward proximity. Many people just have the personality type that drives them to solely play single player games, and there's no amount of game design that will push them to socialize.

But it varies - that's what it was like on my university campus, but I've also been to park events where the age range and social friendliness is extremely varied and wonderful. It is a fascinating and unexpected community.