The reason is U.S. banks have been getting creative with reward programs lately. They entice you with "5% back on gas" or "100,000 points" which all seems very grand. So naturally us nerds want to hack the system for maximum profit.

The reality though is that these programs are not sustainable if fully exploited. Interchange (the fee U.S. merchants pay when you run a card) is under 2%.* A bank can't reward you more than that without losing money. Well, they can make up for it on the interest they charge on debt, but presumably if you're optimizing cards then you're optimizing where you hold your debt too. The way banks make these programs viable is with marketing, tricks, rules, time limits, etc. that make you think you're doing better than you really are.

Smart consumers figure it out. But not everyone bothers to, and so there's enough "dumb" ones to make it possible. Kind of like how gym memberships work.

Long story short: If a service like Wallaby takes off and levels the playing field, then these programs will disappear. So if you're one of those "smart" consumers that enjoys exploiting cards to make an extra buck or two, then you're better off not rocking the boat.

*Europe's interchange is a fraction of that, which is why you don't see this rewards phenomenon there.