You're right - the amount of time dedicated to the startup is a poor indicator of contribution. Unfortunately, everything else is even worse.
Splitting equity according to actual results is fatally dangerous, because you can't possibly know these results in advance, and trying to split the equity after the fact will create a conflict in 99.9% of cases, with a 99% probability of breaking up the team and 90% of killing the startup (the figures are guestimates, but that's the order of magnitude). The whole point of the founders agreement is to agree on everything in advance so as to avoid the conflict situations.
And trying to estimate the contribution of each member in advance, based on their resumes or something is even harder than trying to valuate a pre-revenue startup. Besides, how exactly can you compare different contributions? Suppose, co-founder A wrote 500 lines of code per day, co-founder B generated 50 leads per day, and co-founder C raised the seed round. Who's contributed more? By how much? I have no clue.
Like it or not, the time dedicated to the project is the only objectively measurable parameter (at least in most cases). I learned that after trying other things and having them exploding in my face. Besides, if one co-founder clearly produces much less useful results than the others, then the solution is not to give him/her less equity, but to kick him/her out and find someone better. That's why we have vestings and cliffs and all that.