So if Tyranitar and Scizor are everywhere, both pokemon that can deal enough damage to Rankurusu to make it completely useless for the rest of the match, are "everywhere," then why is Rankurusu such a big problem?
Because stallteams cannot run Tyranitar and Scizor reliably while the offensive opponent can run both Rankurusu and T-tar/Scizor as they work very well together.
Also, have you used Spiritomb? It's not completely useless for stall teams, making an effective staller and spinblocker. It's also an effective user of Will-O-Wisp, and with its typing with no weaknesses, it can provide an effective stop to many physical attackers
All these calculations have been made against a maximum Physically invested Spiritomb
CB T-tar Crunch: 49.7% - 58.6%
CB Scizor U-turn: 42.1% - 50%
Yache SD Chomp Outrage: 44.4% - 52.3% after SD: 88.2% - 103.9%
Gliscor can set up with ease thanks to it's immunity to Burn
Life Orb Doryuuzu EQ: 49.3% - 58.2% after SD: 98% - 115.8%
Roobushin can set up with ease as well considering you actually boost it's power after a burn
Gyarados: Most common version nowadays is the taunt version to stop Nattorei, spiritomb can't touch it.
Dragonite: I'll give you this one.
LO Blaziken Flare Blitz: 54.3% - 64.1% after SD 107.9% - 127.3%
Metagross Meteor Smash: 41.8% - 49.3% -> you induce burn than you die i hope for you it doesn't have Lum berry
Lucario: I'll give you this one
LO Randorusu: 61.2% - 72% in Sandstorm
LO Cloyster Icicle Spear: 52.6% - 62.5% after Shell Break: 105.6% - 124.3%
Choice Scarf Jirachi: 25% - 29.6% -> you may win if you're lucky
I wouldn't consider a pokemon who can only reliably stop 2 out of 15 of Generation V's top physicall sweepers with a maximum defensive investment to be an effective physicall wall. Having no weaknesses isn't worth much, to be an effective wall one has to have usefull resistances or immunities like Gliscor and Skarmory do. To make things worse Resttalk is only way Spiritomb can heal itself but with the new sleep mechanics those aren't as reliable as it used to be.
Have you seen Obi's famous stall team? It utilized Perish Song very effectively to stop last-pokemon set-uppers which usually decimate full stall teams. 8 PP is plenty enough to stop bulky CMers like Rankurusu.
I have read Obi's stall team with much interest and yes, and i admit it was an effective team during Generation IV. However that it was effective back than doesn't necessarly mean it's effective in Generation V
How is 8 PP enough? Take the fellowing scenario: starting with a Heatran-blissey matchup
Turn 1
Heatran switches out to Rankurusu
Blissey does whatever(can't hurt it anyway)
Turn 2
Blissey switches out to Celebi
Rankurusu uses Calm Mind
Turn 3
Celebi uses Perish Song
Rankurusu uses Psycho Shock
Turn 4
Celebi uses recover
Rankurusu switches out to Heatran
Turn 5
Celebi switches out to Blissey
Heatran uses whatever( can't touch blissey regardless)
Repeat this proces 8 times and the Rankurusu user wins, i've done this 3 times succesfully on the ladder. While it doesn't work in a simple proces like that in a real game it usually comes to switching in Rankurusu 8 times which is incredibly easy against a stallteam.