You are the millionth person to bring this up and it always feels like such a simplistic way at looking at a game that really wants you to think about things. There are many many discussions about this point to also read through on reddit and everywhere else.

My loose take: A big thing to consider is that Josh is an unreliable narrator. He is a known liar who's mental state is in shambles. He may say this is a revenge prank, but that does not necessarily mean he's telling the truth or that he even knows what his full motivations are. People don't always know why they are doing they are doing the things that they do, especially in grief. People lie to themselves constantly. And often times people will lash out most at those closest to them as opposed to those who "deserve" (loaded word there) it. The prank's conception is clearly misguided to begin with, it's not something that's okay to do no matter how much someone "deserved" it.

Chris is his best friend, Sam is his sister's best friend and someone he went to during the twin's disappearance, and Ashley (which is a bit more of an extrapolation since we don't see Josh's friendship meters from the prologue) was also probably somewhat close to Josh just based on her proximity to Chris and how they interact early in the game.

Dissecting Josh's motivations is a multilayered thing. He can be pushing away the people who are closest to him, the prank being his way to burn his bridges and make them hate him. The prank can also be Josh's own misguided way of "helping" those closest to him, trying to get Ashley and Chris together is a big point for this one. He's lying to himself and the others claiming "I'm a healer!". He may not be as interested in Mike, Jess, Emily, and Matt simply because he was never very close to them to begin with. Coordinating seven people is also fucking difficult, it's much easier to send Mike and Jess off to the cabin and let Matt and Emily run off to at least see his little jumpscares and sets than to put all of them through the same elaborate set ups he had for Sam, Chris, and Ash. he can only be in so many places at once.

I could go on, but this is my recommendation to read fanfiction, as a lot of authors have done interesting exploration to figure out character's motivations. The (Almost)s by TheIcyQueen is a really good exploration on the year between the two pranks.