my guess it is the way it is because the economics are different for amex and chase

for chase, i speculate the restaurants do pay chase (directly or via opentable) to drive business to them. as such, chase cannot expand as they wish

for amex, the credits might actually be beneficial for them as they own resy. the credit means amex can charge higher af and also attract new signups for their cards. the latter can generate more revenue/profit as people put spend or carry balance on their cards. it also drives customers to resy restaurants, so more restaurants want to be on resy, and they have to pay amex

the 2 points are also self-reinforcing. as more restaurants get on resy, the credits become easier to use and are more valuable to cardholders, so more people sign up amex cards. and as amex's base grow, it drives more businesses to resy restaurants and hence encourage more restaurants to join resy