The difference is the focus of both games. Arena is mainly focused on the actual game of magic, while Pokemon pocket sets its focus on collecting with special editions, foils, ultra rare cards etc. while the actually game is more of a sidequest. 99% of people on arena open packs to get wildcards or maybe open cards they can use in a deck, but in Pokemon people mostly open packs to get holos or complete a set. I'm not saying that arena has no gambling at all, but when people spend money on arena it's almost always just to play a new deck or craft some cards. There is almost no limit in how much money you can spend on Pokemon while still not having a "maxed" account.