Some PvP for the loot, some for the fun, some for both.
One of the point of PvP is, you never know how other players will react, when you attack them and you never know how the PK who attack you will act when/if he kill you.
If you fight monsters, you know how they will react, how strong they are and what loot to expect.
When I still did some RP PvP, I did not kill for the loot, I played a vampire and I really only wanted my victims blood to keep me healthy. Other do kill for the loot too as PK's do get killed and looted too, they need loot, just like you kill monsters for the loot.
I rather die to a PK and get looted dry than to a monster gank and see my stuff decay, because I can't get back to claim it. I had learned always to have at least one suit ready if I die. Even on Siege, I don't die offen and it's sure not everytime I get heavy looted. Some may only loot my loot back, some only kill for the challenge and a few will loot my dry.
We had seen complains here, from someone who tells, they always get dry looted and rez killed. I had dry looted and rez killed a few too but there had been a reason for me to do it and trust me, they did deserve it. With that said, Siege may sometimes have a few grief PK's but they are few.
When I started UO, I was 44 years and female. My sons started to play and soon I was playing too. It was on Atlantic, with dial up from Denmark so lag was very bad and I offen found me char dead to monsters or PK's after lost con.
I loved the game did not hide in town, even when I counted 30 red names, whith hunting ground in east brit, where I did my hunting, lumbering and mining. You may think I would die alot, not really, only a few of them killed me more than a one time but they learned me alot about PvP and how to fight back. Most ended up as friends and after 5-6 months or so, I started to RP a highwayswoman and started my guild, The Dark Outlaws.
Many of my red friends wanted to run with me but I did not want to be in a gank, who just killed and looted their victim. Most time I did run alone, but a few did agree to do it my way, when hanging out with me.
If their had been Trammel, when I started, I fear I would had ended up as a scared Trammel player, who never would go to Fel or to Siege. But I'm born before Trammel and I love the freedom and wild life on Siege.