I think you've summed up the entire point of loyalty programs entirely right here. If every property in the world earned Bonvoy points/benefits, there would be no point to the program. The point to the program is that when I go to Rome (for example), I probably look at the 12 Bonvoy options first, and if one of them meets my needs with respect to location/quality/budget, then I'll probably book it. In return, Marriott offers me points and benefits. If none of the Marriott properties meet what I need, or if I find a better deal elsewhere, then I book elsewhere. The loyalty program incentivizes me to book Marriott properties when all else is substantially equal. Pretty simple.