Can we just say that "uhmazing graphics" has nothing to do with how good a Pokemon is.
Golurk has abysmal speed. It can still be out sped even after one Rock Polish, and if you choose No Guard for Dynamic Punch, it means that it'll always be hit with Will-O-Wisp, resulting in it being checked almost entirely by Weezing; in the higher tiers it would have to deal with Sableye. It's so slow it can't even get a sub up before getting burned, rendering it useless. Yes it's got 3 immunities, but the trade off is 5 weaknesses, and pretty common ones in water, ice & grass. Some calculations will show that Rotom-Frost has a good chance of OHKOing Golurk with Blizzard:
>252 SpA Rotom-F Blizzard vs. 252 HP / 0 SpD Golurk: 372-440 (97.3 - 115.1%) -- 81.3% chance to OHKO
Phantom Force is not a particularly good move either... it takes 2 turns to activate which just means you can switch into a normal/ dark type to take the hit.
And what do you mean fighting types can be a threat nowadays? They were always a threat. In fact they're even less of a threat now because of the Fairy type, and few Fighting types carry Poison moves stronger than Poison Jab.
Stantler isn't a good counter to Mega-Gengar. Yes, it's normal type, but it's got crap defenses and is out sped by regular Gengar, let alone Mega, so Gengar can set up substitute on it and any status move is therefore useless. If you want to use a Psychic move, surely Girafarig is better because it receives STAB on Psychic moves. Considering that Purugly outspeeds Gengar and has Shadow Claw, while Persian also outspeeds Gengar and has Technician Bite, it's not as if Stantler is the only NU Pokemon ever capable of doing anything to Gengar. Just to prove my point:
>252 Atk Life Orb Technician Persian Bite vs. 0 HP / 0 Def Gengar: 257-304 (98.4 - 116.4%) -- 87.5% chance to OHKO
Even if Stantler was a viable counter to Mega-Gengar, it should be able to do more than just counter one Pokemon. While Mega-Gengar is popular over wifi, it's not as if it'll appear every single battle. If it doesn't appear, then Stantler is near to useless if its moveset is geared towards dealing with Mega-Gengar.
I'm not saying they're bad Pokemon, I'm just saying that they have a lot of limitations compared to their advantages, and that's why they're in NU. Even other NU Pokemon outshine them, let alone RU/UU/OU Pokemon. Purugly, Persian and Miltank are better normal types; they either have higher overall stats or better speed/ attack/ abilities. Stantler just doesn't cut it with its mediocre stats. As for Golurk, Mold Breaker Scarf/ Band Sawk out classes Golurk due to its ability to hit Rotom-Frost, and Golurk has no boosting moves for its attack, unlike say, Shell Smash Carracosta, Curse Muk, Bulk Up Gurdurr etc., so its got stiff competition as a physical sweeper. It does have a niche in being able to set up Stealth Rock and spinblock simultaneously, but that's about it.