Most point based 'ranking' systems like these are practically cosmetic and changing the amount of points given for a win/loss wouldn't really change the matchmaking much at all.
Even if you changed it to be truly zero sum, all it would do is compress more of the playerbase in the bottom tiers early on, it would take longer for small differences in winrate to translate to placements in different tiers. You'd still match with the same people. That 'gold' player you think you don't deserve to party up with? Maybe they actually barely break 50% w/l, and in the new system they'd stick around in bronze with you, or silver.
As anyone who has interacted with systems like this can see, the point system is pretty lenient and fake especially in the lower tiers, it lets you 'climb' even with a 30%-40% winrate or something like that. I assume the real reason is to make it less intimidating for casual players, and give people a sense of progression even if they are below average. Frankly, they need all the help they can get with how unpopular PvP is in this game, if you make it even more scary looking with zero sum rankings even at bronze, you'd just chase even more people away.
In the end it doesn't really matter, only the top 100 rankings matter at the end of day - and those are truly zero sum, for every person that gets put into it, someone in there gets booted out.
