Thing is that while Kyogre may take quite a bit of supp to function properly, the investment always worth it as it gives so much back in return. When properly supported, it becomes one of the best shields against Groudon in the game (not necessarily for itself, but between itself and the rest of the team thanks to Primordial Sea), which no other Pokemon can really claim with the same amount of support. In the event that an Groudon receives a similar amount of support, the matchup advantage usually goes to Kyogre as Groudon desperately needs the sunlight to not get destroyed and cannot OHKO Kyogre.
Kyogre's extremely high base Special Attack and the ability to downright blow up so many things in the game often means it doesn't require much, if any, Special Attack investment to be successful. Physical defensive investment is ideal in a lot of cases so Kyogre can take multiple Pblades if it has to, ensuring maximum Primordial Sea uptime. Kyogre can even take a combined LO Adamant Talon BB and Max Atk+ Groudon PBlades and survive, while eliminating both with Origin Pulse.
Kyogre has seen successful pairings with Groudon (obviously), Xerneas (won St Louis regionals), Yveltal (won NJ MSS which was kindof stacked), Dialga (Wolfe), Kyurem-W (St Louis t4), and if you count the earlier season, Ho-Oh. As for Giratina-O, a japanese player made it to 1900 last season on the special ladder with Kyogre + Giratina-O. You don't have to pair it with Rayquaza to be successful.