Because the entire point is to hurt the casuals. That IS the gameplay.
Yeah, I guess so. To me this kind of thinking is as alien as Noaani's defense of hardcore pve

I guess I grew up in such a "proper pvp" bubble, that both extremes are too far from me. Or at least that's how I look back on my history in L2. There's probably some nostalgia bias too.
Actually, I realized I gave too short an answer.
It's not just griefing.
If I kill you and take half your stuff, I am twice as far ahead of you as I would be if we both gathered our own stuff separately.
Other ways this works well, especially against newbies who don't have guilds yet:
Extortion.
Protection Rackets (related to the above but different).
Gaslighting (this one is complicated to do before you have power on the server but VERY easy once you have it, especially in a game with anonymous alts)
Creating double agents and spies through the above.
Good cop/bad cop emotional manipulation.
Economic blockades and shutdowns explicitly targeting the weaker players.
Piracy against players with large investments that were used to their area being safe.
These are just the ones off the top of my head.
I believe all of these are intended design, and it would be incredibly beneficial (in some cases required) to have a geared alt for these purposes.