After doing some research I found that the main reason for these poor rewards in Europe is due to Europe changing the regulations around how much card processing companies (Mastercard, Visa etc) can charge merchants. Less profits for them at the processing level means less rewards for customers.
The upside though is that businesses in Europe (with the exception of Germany) seem to be more okay with paying by card for everything. Meanwhile, businesses in the US (especially smaller ones) are much less pleased, so using a card for smaller transactions is still kinda frowned upon.
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Rewards cards are a huge economic distortion. What's the point of charging the merchant more to give the customer a kick back later? It makes the customer want to funnel more purchases through the card, meaning more transactions for the processing company to skim from. Otherwise it would be much more efficient if the merchant kept prices lower.
This assumes they will actually lower prices if interchange were capped or eliminated. Somehow I have doubts as to whether that'll happen (but as mentioned above, they may become more okay with card payments in general).