Perhaps people do so because they see their console of preference as the best and do so by either posting mostly positive comments about their favorite brand and/or giving negative comments with little substance to the competition.
There's also a 3rd case of said posters making threads to put their opposing companies in a negative light, followed by others actively participating to bring down that company. Perhaps an example like 'NES Minis are impossible to find, shame on you Nintendo for being so terrible to your loyal fans' with following comments like 'This is why they're a bad company and should leave the business imo'. That's just no good.
However, it IIS possible to give criticism and have thoughtful open discussions on a subject, explaining why something is good or not good. For example, if someone were to say 'Nintendo is the best company in the industry' of course that is open for debate with several reasons against that comment, or some supportive ones. But, if there's a thread such as 'This list of games shows why Sony is doing excellent this gen' and you follow with 'Only a few of those games are good imo, Xbox/Wii U has a better list' than... that's not really a good comment.
So yeah, it is possible to go into a thread, or make a thread, on something you're not interested in, but still have something meaningful to add, but there are times when people just go in to take a jab at companies they don't usually care about.
Dance my pretties!
The Official Art Thread - The Official Manga Thread - The Official Starbound Thread
