Because they can. Have you played GTAIV? This must be all be new to you.
I played GTA IV. I went into GTA IV multiplayer a couple of times, maybe for a total of 10 minutes. Got killed a few times. Saw it as a mindless, stupid pointless waste of time. That's my expectation of multiplayer or online. So I knew what GTA0 was going to be like. And it is. The same stupid sh*t.
The difference is that GTA IV - TBOGT was fun to play in single player, for years. But in GTAV single player, there is nothing to do after the short half-assed story. So I go online, usually Solo. When I let myself drop into a public game, the results are entirely predictable.
Basically all multiplayers have the mentality of zombies. GTA0 is a zombie game.
GTAIV was great fun if you turned friendly fire off because players collaborated together like doing stunts at the airport or simply going on a huge rampage together.
GTAV needs the same option. RDR had it so I haven't a damn clue why it's not in this game.