That's not the full reason.
Half of the time, people don't go %pen anymore, and Lw+Void did exist in the past aswell, yet def stats felt meaningfull.
Also, you can't add flatpen to that calculation, if you wouldn't have the MR from the item, the flat pen would reduce base MR instead.
50 base mr -32 pen = 18 remaining
50 + 55 mr -32 pen = 73 remaining, which is 18+55 so the effective increase in MR from the item stays the same.
Anyway. the main issue is that the gold values compared to off items is just a bit low, damage values scale way to hard and way to many champs/items/runes use anti tank mechanics.
There are so many champs with either %pen/shred, truedamage or %hp damage, and if not there is always conqueror, botrk, liandries...
3 items
lvl 11(important for steraks gold value)with
sits on 3622.5+2915+2900 = 9437.5g worth of stats.
On top of that we have the Rage effect and a very powerfull spellblade from trinity, the singletarget on-hit, aoe-on-hit and the massive active from hydra aswell as The huge shield+tenacity from Steraks.
Also, steraks will be worth another 560g at lvl 18 thx to higher base ad, increaseing total gold value to 9997.5g
Now a
with 3 items
has 2333+3000+2937 = 8270g with sunfire aura, 30% increased healing and the warmongs out of combat regen, which all together imo is not even close to the passive/active from even just trinity+hydra which together add easily 600+ burst damage with dps potential on top. you'd need 15sec with sunfire to reach that damage. Just that a champ like irelia will easily get 5+ more trinity procs duering that time for 1k+ damage total just from the trinity procs.
Sure the tank items are cheaper, but at full built that doesn't matter and they just don't have any more expensive options, and they usually have less gold cause they have a harder time farming up past the early lane when most others outscale them 1v1 and their waveclear needs some time. They also rarly get bonus gold from solokills compared to more damage focused champs and usually have a harder time getting plateings in the early, so usually they would still reach their 3 item spike at a similar time as bruisers hit theirs.
And for the sake of completion:
with
= 2664+2597+3000 = 8261
with
= 3983+3800+1948 = 9731
with
=3331+2789+3654 = 9774(with the pen from morello and the extra 10% cdr on ludens)
So only assasins have similar low gold valeus(and costs) to tanks, but with ALL the gold straight focused on AD, pen and CDR, all prime stats for them, while tanks split some gold between mr and armor makeing their effective gold value vs someone like riven even lower as she doesn't care about any of the mr.
Also, quite some of the tanks gold value goes into hp/5, which is a comparable weak stat outside of laneing phase. In standoffs it's void because warmongs passive already does that job, and in direct fights it's not a very usefull stat with how short they usually are and how hp/5 is tuned.
Another huge issue is how much magic dps is in the game. Back when defense felt like something, magic damage was rarly dealt as dps but usually as burst with decent downtime, which ment most magic DDs avoided hitting tanks and saved their CDs to burst a squishy if they get the opportunity. As a result, 1 mr item usually was enough paired with a good hp pool and good hp/5, allowing to focus more on armor.
Now Mages haf so little CDs, half of them(the meta ones anyway) are dps based like cassio, ryze, azir and in a way corki, with adc like kaisa with lots of magic dps that even has a big %hp component, but unlike kogmaw that's not her niche and big strength but just part of her burst + mobillity + poke + dps do it all kit, there is just so much magic damage vs tanks aswell that they usually only can decide wether they want to die from magic damage or from physical damage.