The reason is that the weight is as you said relative to height. I’m 6’4” and weigh 180, a dude that is 5’10” and 180 appears to have more muscle mass and also lifts slightly more on squat and bench. Despite this, the weight is irrelevant when it comes to real world application as both my and the other dudes height and weight allow us to do slightly different things. Therefore it’s not worth mentioning since it doesn’t matter much at all. Your height is your height and you can’t change it.
your weight by itself isn't always a great metric of how much muscle you actually have unless you have a picture to go with it or a precise body fat percentage. I agree that height doesn't really matter since it's your change in bodyweight that really matters. But if you're trying to flex your strength on a taller person... by all means