Maybe, just maybe, it's because "the heavyweight process/rituals required to be accurate" might not be productive, and because being startups, and therefore small organizations, it's easier for everyone to know who's pulling their weight and who isn't, therefore "heavyweight process/rituals" add nothing and cost too much.
Mature organizations tend to "implement the heavyweight process/rituals required to be accurate" precisely because they are too large for everyone to know everyone, and so senior management loses touch with reality and starts feeling anxious about whether their R&D spend is yielding value. This is totally understandable, and we have to have empathy for executives, but there is tremendous danger in this approach. How many mature market leaders have had their lunch eaten by disruptive innovators (invariably startups)? And why? Maybe those "heavyweight" processes kill innovation! That urge to accurately measure what the org's devs are doing can be counterproductive.
All measures but one (so far) are gameable. So far only KTLO fraction, which one should couple with promoting a management culture that allows subjective value judgements to make it up and down the chain. Management of knowledge work essentially is a social problem, not a scientific one.