The biggest reason I reckon is due to it being more exciting to get to see the characters meet and fight each other or be in awe at each other's skills. Like viewers get excited to see how the new characters in every arc will react to the main characters' powers (eg "this is a loser pirate crew" to "holy fuck this is a strong guy) - same kinda thing. Even in HxH, they don't exhibit any crazy powers to start with, but it's exciting to see how they (Leorio, Gon, Killua, Kurapika) will gain each other's respect.
Also, it's easier to write individual backstories and then have the characters meet and develop their relationship in the backstory I think, rather than having to write a backstory of how they know each other/how their pre-existing relationship developed and then having to also develop it further in the present. It gives the writer time to develop each character without having to think about dialogue or having to balance multiple strong personalities. It's also a quicker way to get the cast together, rather than having to spend lots of eps/chapters on developing one set of friends at a time. Plus if some characters in the main cast group were friends beforehand but others weren't, then you have to write that into the interactions which is more complicated than them all just being strangers. One example would be if the long-time friends have private conversations behind closed doors, away from the other cast members. The cliques would have to be either maintained or slowly broken down, but either way it requires more thought than just having a bunch of strangers meet up and slowly get to know each other all at the same time.
Wow, what a sad ending.
