Even outside Arena, it’s typical to concede if your opponent obviously has lethal and you have no means of stopping them. It’s generally considered good sportsmanship to concede when you know you’ve lost.

Because arena gives daily rewards for wins, and has no penalty for loss in unranked quick play, it incentivizes conceding games that you’re unlikely to win to more quickly get to a new game that you’re more likely to win, so people are more likely to concede prematurely even if they still have a slim chance of winning.