I'm just asking so please don't judge me, I care about sales too.

but they don't fight about because movies have more sales than anime.

Cyndi Lauper > Madonna > Lady gaga

Sales of a game matter a lot in the industry.

Low sales can lead to a lack of support for certain consoles, no more sequels or games from a developer, and other important factors that can affect the industry either positively or negatively.

It depends where you're looking. There are going to be a group of people in every entertainment medium that cares about the numbers they do.

But why we use sales to fight (not we, but fanboys)

Sales are objective facts. They can't be bought, bullied, reasoned, or negotiated with.

Nah with games only people like us care...most people buying games dont give a crap.

The music industry is heavily orientated around charts.

Yeah I know, that's why I said "more than any other entertainment business" I use movie forums and music forums and the percentage of fans arguing or fighting about sales is less or none. Like I said, I'm not talking about vgchartz, in general.

Again, sweeping generalization of people you don't know. When was the last time you've been to a movie/anime centric website or forum?

Fanboys are present in any medium and you can bet your ass that they will grasp for anything that will make their fandom look better than others. That includes sales numbers and all fandoms.Justin Bieber fans proclaiming that he is superior because he sold more albums than X musician. Sound familiar?

Again, just because your experience is mostly on the gaming side doesn't mean it's happening less in other mediums. Look at gaming websites and how often they publish sales numbers and then look at movie websites and look how often they publish sales.