Robin Hanson tries, and so do a few others. But yes, in general people don't think this is necessary, and (here I go) that is greatly to the discredit of MWI's advocates. If ever I wanted a simple way to categorize all the different shades of opinion about MWI, while also demonstrating that almost all of them have deep problems, I need only organize them according to how they think about the Born rule and the origin of quantum probabilities.

Perhaps the most reputable version of MWI is Gell-Mann and Hartle's consistent histories formalism. This formalism gives you a prior for the different histories, but no attempt is made to "ontologically interpret" these probabilities.

Then we have a "no-collapse wavefunction-realist" interpretation which centers on decoherence and on the appearance of probability-like numbers in reduced density matrices. This is a "folk interpretation" among working physicists, and like all folk theories, it does not come in an authoritative official form, usually hasn't been thought through, and so it's hard to simply rebut. Instead you would have to ask questions like, is there a preferred basis?, and, what makes those numbers probabilities?, and see how the individual physicist responds.

Then we have people who say that there's one world for each possible outcome, but that some worlds "exist more" than other worlds, or are "more real" than other worlds. I wonder if that answer has ever been tried in a court of law? "Mr Casino Owner, the ball keeps landing on double-zero more often than it ought to." "No, that's not true! It lands on all outcomes equally, but the double-zero outcome is more real than the others." It's an expression rendered meaningless by self-contradiction, like the round square; the result of trying to reconcile an ontological commitment to the equal reality of all outcomes with the inconvenient fact that they don't occur equally often.

Then we have the "decision theory" approach to deriving the probabilities, which I'm glad to see is being met with some incredulity, here on a site where people care about decision theory and know something about how it works; but which nonetheless has somehow acquired a reputation as a serious and important approach to the question.

There would be still other schools of opinion on this matter. And then finally, hardly noticed, off in a corner by themselves, are the MWI rogues and renegades who are trying to explain the predictions of quantum mechanics regarding the frequencies of events in the multiverse, by exhibiting a description of the multiverse in which the frequencies of events do in fact match the probabilities! (And then we have the "MWI public", who naively think that Many Worlds means that there are many worlds, and who don't know what a mess the interpretation is in, when you look at its technicalities.)

I would say that explaining quantum probabilities in terms of event frequencies in the multiverse, is the only sensible way to seek a multiverse explanation of QM; the fact that "deriving the Born rule from counting" is very much a minority concern in the real world of MWI studies, is a symptom of something very wrong with the whole "field".

ETA I include deriving the Born rule from a measure, as a form of "deriving the Born rule from counting". But note, talking about measure is not the same thing as explaining its form. Saying that "measure is concentrated at this world" doesn't explain why it's concentrated there, or what measure is.