Console Wars: Why do they exist?

I always thought it was because unless you're the type who owns each one, you're basically going to try to justify your purchase so you don't regret the one you picked.
Because video games are expensive and not everyone can afford everything.

I haven't had a strong preference on a console since the SNES vs Genesis days. The ubiquity of multiplatform games in the modern age has made console wars seem even sillier than they used to, now that everyone is playing like 99% of the same games, anyway. At least it used to be wars about different games, now its wars about pixel counts on the same game.
I see two facets to it.

People reaffirming their purchase. Mostly kids who can only choose one.

People who openly voice their expectations as a consumer.
Cause people like to argue and like to think their side is best.

If you think this is special to gaming... ha!! Sports, phones, tvs, cars...

Anything that has people passionate about it has people arguing which is best. People like to pick sides and think they have the best side. And honestly, I think sports fans can be way worse about it.

Honestly, I think as long as you can be civil about it and not be so biased that you cannot possibly see any advantages to the other side/disadvantages to yours, there is nothing wrong with it. But I like arguing ;).
Why do you think console wars exist?

Usually because some kids can only get a console and want to justify their choice as much as they can. Doesn't help when those companies have an history with previous "wars".
This kind of stuff is not limited to consoles, but the fact that it's hardware makes it easier to push the war arguments.

Mature people with only one console live well with it.

you mean, why do people have different opinions? because different folks different strokes.
If you think console wars are just that, you probably like those wars and take part in those.
Consoles wars as in between companies in the market or between fanboys on the internet? Because one is significantly stupider than the other.
I'd like to say it's because there are a lot of younger people on the internet but I have met some serious, balls to the wall die-hards on both sides and it scares the shit out of me.
Because humans are competitive and these companies are competing for both mind and market share. People get very defensive over anything they invest time, effort and money into. Examples: how to raise your kids, your favorite sports team, literary criticism, etc.

Also, some people are just really stupid and like to fight over anything.
It's really just human nature to think your team is better regardless of the form that team comes in. Then, add in anonymity and people will take things to animalistic extremes as a matter of insecurity or ego. That's really all it is.
Justifying a purchase aka post-purchase rationalization. It's not exclusive to consoles. Look up car forums. Same there with BMW vs. Audi.
I'm surprised you didn't use Chevy vs. Ford. That seems to be the loudest argument amongst car fans (to the point of having Calvin pissing on one or the other's emblem).

(Personally I like both, it's Chryslers that suck!).


Phone reception is more important to me than human rights
Because damn every product has a fanbase. Consoles are no different.

In mobile phones...of course you got Android vs iPhone. In Android land you have:

Root/ROM vs non-root/ROM
Stock Android vs custom UI
App vs app

When I saw people fighting about which home screen launcher/app was the best...I knew there was no level to what some will fight over.
Same reason why people care about "their" team winning in sports. Even going so far as to insert themselves into the conversation like they had anything to do with anything. "We won." "We can't afford any more injuries." "I can't believe we didn't win last night."

There's just something inside people that makes them like something and compete against others who like something else. Especially when you can easily view those things as being in direct competition.

And like sports, it's generally harmless, but it can go too far.
The same phenomenon occurs in other industries, not just videogames. It happens with phones, sports teams, etc.

People want to feel like they belong to a community, in this case it's a community tied to a brand. They begin to self-identify with the brand, and any attack on that brand is interpreted as an attack on them.

Basically people allowing commercial brands to dictate their identity.
Because it's not enough to prefer something and show that preference by buying it.
You have to prefer something, show that preference by buying it, and then justify your preference by talking about it to other people.