Yeah the reason why biomass is used instead of number of individuals becomes rather clear when you consider the following:

• what counts as an individual? is an unborn already an individual? (that one's a heated debate, as you can see by the abortion debate)
• if unborns are individuals, then at what age are they?
• if they are from the moment of fertilization, then some animals, like spiders or frogs (idk any mammal examples, but there might be some), might lay a shitload number of eggs, like a million or sth, and it would drive up the number of individuals dramatically. But it would be a bullshit metric, because 99% of these individuals are never gonna survive a single year on earth. so it would be utterly confusing and misleading.

Going by mass solves all of these problems because it's more clear and more direct. And on top of that it has the nice side-benefit of also giving an estimate of land usage. Land usage is roughly proportional to biomass, so measuring biomass is meaningful to estimate land usage as well, and that one really matters as that's the limited resource that you're trying to distribute among all species on earth.