The trouble is, they aren't, not even close while leveling jobs. I know I know, everyone here on the forums can pull every dungeon right from zoning in it up to the last boss and easily tank it all while the healer picks his nose, I get it. For sprouts learning the ropes, nada.
I've leveled all my tanks in dungeons, I don't do PotD because it gives me brain rot after 47 seconds and FATE grinding is a long lost relic of a bygone era, so I have a decent feel for what a tank can and can't do. W2W pulling in expert or 50/60/70 roulette is not impressive, we over-gear it by a mile and we have lots of abilities to play with. Leveling dungeons are a different story all together, especially if you are leveling through it yourself and not getting that juicy level sync ilvl bonus.
The trouble is there is a tendency for the player base to tell all new players that tanks should be going big or going home so we have what I had the other night, in a party of 3 sprouts the tank pulls the entire first room of Aurum Vale, then blames the DPS for being slow when we wipe and the healer ask him to calm down. He lost health so fast I doubt I could have saved him on level 80 BLM never mind level 49. The RDM was decent, and I've been playing BLM so long I can remember timing server ticks to triple cast flare in Wanderer's Palace when it was the current Tomestone grind, and it's not hard to get BLM AOE right.
It took Asmongold one pull of Sastasha before his chat started yelling at him to pull bigger, fortunately he has more than enough MMO experience to know when to bite of enough so he can still chew, and balls to tell his chat to stfu.
In leveling dungeons and with experienced roulette tanks and healers with sync bonus, and strong DPS you can indeed pull quite big, but if you have sprouts, especially in current dungeons for their levels, one or two packs is usually more than enough depending on the dungeon, W2W just ain't happening and that's ok.