Well, one reason is because RBF is probably reasonable at smaller fee-rate ratios. Eg with current rules I can RBF a transaction repeatedly, paying just 1sat/vB more each time. That's more reasonable if I'm paying for the entire bandwidth uses each time, in the sense that the total fee goes up by 1sat/vB each time, even though the fee-rate went up by, say, 1%

With One-Shot RBFR if I could do a replacement with a fee-rate increase of just 1%, during rising fee-rates I could use the "big/small" replacement technique over and over again, using up a lot more total bandwidth each time. Setting the minimum RBFR ratio to something more like 1.25 makes the fee-rate I'm paying grow a lot faster.

But certainly, if one-shot RBFR was the only mechanism implemented, you could still do most of what we need fee bumping for; there's debate over whether the existing RBF rules are reasonable. Maybe you shouldn't be allowed to rebroadcast a transaction hundreds of times, by just increasing the fee-rate a bit each time?