A prime example of this is CSR ur points can be cash, transferred to partners or 1.50x on their travel portal. Cash has the least redemption value.
That is to say that travel cards are designed in such a way that for their rewards, a travel redemption is higher value, or at least it appears to be a higher a value. But that is not to say that using optimized cashback instead and using that rewards cash to either find a discount deal or bargain for one (oh yes, those exist for cash sales) can't be just as cost effective or more so. Another benefit of a cashback card is that you can use the cashback to pay for your credit card bill for the travel, but because you are using the card to book, you will also earn cashback on the booking. You don't earn rewards on the points portion of a trip booked with points.
"Travel" cards also tend to come with an AF more than do cashback cards.
Not saying travel cards can't be an important and simple way to feel rewarded, and if redemptions like first class tickets on international flights is valuable to you, then by all means travel cards are the way to go. But for bargain hunters, I tend to be partial to cashback.
I don't always see a strong correlation between the value I get from a card and the amount I spend on it. Spending matters for to some degree, but I get more value from bonuses and perks.
I spend a lot on Prestige, but not a lot on Platinum or IHG, for example.