When you're a salesperson, you don't have to agonize over how to illustrate that you've "Decreased Customer Churn Rate by 10%". When you're an assembly line worker, you don't have to find a way to figure out how you contributed to "produce 15% more widgets". You either accomplish these things or you don't - they are literally descriptions of your performance in your job. Either way, nobody cares how you did (or didn't) do it.
When you're a developer, how are you supposed to show that you've helped "increase customer value from $N to $N+5"? Because "shipped the new version of the message queuing system" is not in anyone's list of key results.
OKRs feel like "organized SMART goals", and so I have the same criticism of them as I do of SMART goals: they're just another way of conceptualizing goals around things that are already easily and directly measured. No one has made any progress in quantifying the contributions of roles that don't have direct percentage impacts on dollars earned or dollars saved.