Okay, here's a simple way to explain why we use numbers.
Yes, the numbers are a bit arbitrary, but their only purpose is to show how severe one character's advantage over another is.
For example, Marth vs Ness can be viewed as anywhere for 80:20-90:10 in favor of Marth. What does this mean? It means that as a character Marth ***** Ness. It's not something the Ness player should ever be expected to win if the players are even remotely close in skill, it's just that bad.
Marth has more range, racks damage faster, kills earlier, and in general outclasses Ness in almost every category. And on top of all that, he has a release grab infinite, that chains into down smash or f-smash for the kill.
If I had to express this severity of advantage on a 0-10 scale, I'd give Marth 9/10. Now to put this as a numerical ratio comparing Marth's advantage to Ness's disadvantage it would look like this
9:1
And that's it. Now when I want to describe how bad Ness is against Marth, I can just say it's 9:1 Marth's favor. I don't have to list all those reasons why, I can just list that ratio, and everyone will understand that Marth ***** Ness. If you disagree with the number, then you look up the reasons yourself.
The numbers are only there to serve as a quick notation. When you compare numbers, you're actually comparing the severity of advantages in each matchup. A 1:9 matchup is much harder than a 4:6 matchup, and the numbers just represent that difference in difficulty