Why Is So Much Emphasis Placed On Belts?

Yeah, but the other 5% counts. On the other hand none of the never-won-a-title boxers were called the best because they never won a title. So to make my point, it's not only the title but what you do afterwards: doing crappy defences (Erdei) or bargain a great fight and seal your legacy
And most boxers who are being regarded as the best, have at least held on a title... but indeed not athe beltholders are great, but thge greatest were mostly (former) beltholders!