Right, but the escape velocity is ~ radius * sqrt(density) anyway, and if you look at the equation for the Earth Similarity Index[0], each term has an associated weighting. So why include escape velocity explicitly, rather than just changing the weighting of radius and density? The weightings seem to be quite specific numbers too, so I imagine there's a methodology behind it, I was just curious as to the logic. (Also, e.g., why is the temperature weight 5.58? Wikipedia says the weighting is to 'equalise their meanings', but it's not clear to me what that means.)
I think the formula that Wikipedia gives isn't used that often for most exoplanets. Oftentimes all you know about a planet is the mass, radius and some information about temperature or at least energy flux to its surface, so they use those at first.
I guess the parameters were chosen by some curve fitting procedure to make sure it gives 1.0 for Earth, 0.7-ish for Mars, 0.2 for Uranus and Neptune which are basically the most "not like the others" in our solar system and so on. It's not really a measure of habitability or anything like that. "Equalizing the parameters' meanings" presumably has to do with mapping their relative variation (and the effects of that variation over various observable parameters) over a smaller range.
[0] https://en.wikipedia.org/wiki/Earth_Similarity_Index#Formula...