Most of the people in pvp are new players. 90% of the games I've played in pvp were people with less than 100 games played total and zero upgrades. If there were more people with good pvp decks, it would be different, but as it is now, I only ever see new/weak players in pvp. Also, I can't go afk during a pvp match for 20 minutes whenever I want :p (which I do all the time playing FGs/platinum) There's also the issue that people, when they are losing in pvp, will often just disconnect and make you wait for them to timeout, and then you are just playing their AI anyway, and with the contents of their decks, that usually ends up being about ai1.
In the arena, people make better decks. A bunch of them are still pretty crappy, either due to lack of deckbuilding skill or lack of upgrades, or because the card the oracle gave them sucks, but even a crap deck is tough to beat when it has 2x draw, 3x mark, and a pile of hp. The rewards in the arena are also much nicer than pvp; more gold, rare spins, and even regular spins are better because arena decks usually only have a few types of card, whereas n00b pvp decks will have one or two each of many cards, making it harder to win one. (and less likely to be good if you do win one)
If I ever saw some of the good platinum deckbuilders in pvp, I might change my tune, but it's extremely rare that I ever saw anyone with more than a few thousand points score in pvp, and usually it's a few hundred or so. I used to play a bit more pvp, but now I almost never do because of the reasons I've stated.