they are good enough options and many people see success using basic thalk-type cores, and other similarly popular cores.
eventually variety in your teambuilding is required in an attempt to not get blatantly counterteamed in a tournament format. However, standard teams are standard because they can generally see some degree of success unless explicitly counterteamed. as long as you aren't painfully predictable and keep bringing same teams with same counterteam leverage, there's no need to mess around with much less viable mons. yes you can make mawile work, probably. but you don't have to use any of those stuff to be successful.
I actually went to the DOU Seasonals thread to see if your claims were true:
http://replay.pokemonshowdown.com/doublesou-364023411
p2 brings a very standard looking diancie bish talon keldeo core.
checkmater also brings the standard mega gardevoir tr core consisting of volcanion, scrafty, amoonguss, etc.
http://replay.pokemonshowdown.com/doublesou-364016497
checkmater brings standard™ kanga azu jirachi core team
p2's team doesn't exactly fall under super standard archetypes but these are all pokemon that you wouldn't be surprised to see on any team, feel free to point out if im missing something here ofc
http://replay.pokemonshowdown.com/doublesou-363383503
dawg brings kanga double genies amoonguss. if it's not standard idk what is lol...
samvgc uses diancie hydreigon lando amoonguss which makes for a very standard core. if you don't think this is standard...
http://replay.pokemonshowdown.com/doublesou-363388530
now dawg uses basically the same team which kind of is a testament to how standard a core this is. sets are diff but ofc w/e lol
http://replay.pokemonshowdown.com/doublesou-363392774
this time samvgc again uses a very similar diancie hydrei lando rotom core, and mind you rotom was on the previous 2 diancie teams too, everything looks quite similar here. that's because they're standard.
i could go on but it'd just be wasting our time. I do understand the benefit in letting new players mess around with their new gimmicks and teams and underused Pokemon. It gives a lot of johnnies large amount of motivation and gets them invested in the game, and you'll see why some stuff just work with less effort. but you don't have to use them, and they certainly aren't straight up better than standard™