Because this game has been and always will be one of damage.
How do you take objectives? You fucking do damage to them.
How do you get gold? Well, you're a support, or you fucking do damage to creeps and monsters.
How do you win the game? You do 5k damage to the nexus.
How do you force your laner back, or kill them? Guess fucking what? You do damage to them.
EVERY source of solo gold beyond passive generation is from doing damage. Period. The end. If you want items, which you need, especially as a tank-- you need HP, both resists, some CDR, and usually some form of sustain/regen, well, guess what? All of those items cost gold, and as such, you need to do damage to get them.
Tanks no longer have the minimum damage required to adequately survive solo lanes to get get the gold they need to then adequately survive teamfights. Tanks need more gold, to get this gold, they need to do more damage.
All the ADC's would bitch, whine, and moan if we just gave the stats purchased from items to tanks outright ("muh statstticks* -while conveniently being the single most stat stick based class who literally wins almost purely because their damage formula is multiplicative and they get so much more power and gold efficiency out of their stats than any other class in the game bar none.) so, tanks need more damage in laning.