Arceus-Rock is an used pretty less as a Rayquaza-Mega check as Tyranitar exists. Yes, it is underrated, but just a bit and, in my opinion, is used as much as it deserved. This isn't ORAS now, mate, Arceus-Rock was legit one of the best Arceys formes back then when you had the check Life Orb Ho-Oh and Yveltal, and when V-Create Rayquaza-Mega was far more common than Earthquake. There's a reason why the the Tyranitar is higher in the Viability Rankings than RocCeus. Tyranitar competes heavily with Arceus-Rock for its defensive niche. Tyranitar has the added Dark-typing, which also gives it Pursuit, so it has an edge against threats like Gengar-Mega. If you're running Rayquaza-Mega + Tyranitar, and if the opponent send in their Gengar-Mega to revenge kill your Rayquaza-M, then the Tyranitar can easily Pursuit trap the Gengar-M, so the opponent'll actually have to play some games if they don't want to trade Megas. Even if you sack, it'll not go in vain as the Tyranitar would revenge kill the Gengar-M easily. As Tyranitar has access to a free itemslot, it can run Shuca Berry to act as the closest thing to a Rayquaza-Mega counter which you can get, whereas Arceus-Rock is left with a restricted itemslot. Tyranitar also has the added benefit of checking other scary offensive threats like non-Sunsteel Strike Necrozma-Ultra, Gengar-Mega, Yveltal and Arceus-Normal. Foul Play does solid damage to almost everything Tyranitar is relied on to check, OHKO'ing Necrozma-U and Gengar-M and 2HKO'ing a boosted Arceus-N. Also, Sand Stream's boost to its Special Defense allows to effectively check some powerful special attackers, almost all of which I've mentioned above. Now if we come back to Arceus-Rock, another of its big issue is that whenever it comes in, you're going to be drained of a lot of your momentum. Things like Groudon-Primal and Zygarde love to come on RocCeus, and they can wall it pretty easily depending on its set. Also, common Pokémon like Kyogre-Primal, Groudon-Primal, Necrozma-DM and Marshadow force it out pretty easily, and Marshadow can Pursuit trap a very weakened RocCeus too. Even as a 252/252 Calm Mind Arceus, it's usually outclassed by DarkCeus or FairyCeus, as they do the same job, but better as they can setup comparatively easily than RocCeus. Rayquaza-Mega can also run some common coverage moves like Earthqake and, on some Scarf sets, Surf too, and it's hit very hard by Life Orb Draco Meteor. RocCeus can be easily dispatched and is usually a one-time switchin to Rayquaza-Mega, as after they know that it's in your team, they can make some witty doubles and take advantage of the momentum drain.
In the Unicorns vs. Temp AGPL match (for reference: https://replay.pokemonshowdown.com/smogtours-gen7anythinggoes-571180
don't mind the nicks), you'll be able to see how easily RocCeus was overwhelmed, that also by Ferrothorn, and couldn't keep itself healthy enough to face Rayquaza-M. Even though Temp sacked the Rayquaza-M, it could have had taken a kill every time it came in if he had prevented Stealth Rock and had predicted the Pursuit from Marshadow, whereas Tyranitar was able to revenge kill Uni's Rayquaza-M through Foul Play. Even though Uni was sacking the Ray-M to pave way for the FairyCeus to setup and sweep, the main point here is of the RocCeus. Without enough support, it can be trounced easily, whereas Tyranitar doesn't require that much of extensive support. Tyranitar + Kyogre-Primal, Tyranitar + Skarmory + Chansey, Tyranitar + Groudon-Primal, the possibilities are actually a lot.
I hope I've now made it clear why Arceus-Rock is used less than Tyranitar.
(And about the Alchemist part, they aren't such a good player, have played them a few times and personally I feel that they wouldn't fall in even the Top 15 players right now. The ladder isn't a true representation of the "best" players, try to keep that in mind, the USUM ladder is pretty dead right now. Peace)