bork: 8% hp as phys damage, 100 magic damage active
duskblade: 320 phys damage 18 lethality
razor: 4% hp phys damage
whisper: 35% BONUS armor pen
cleaver: max 24% armor reduction
youmuu's: 18 lethality
Shiv: 160 magic damage
rfc: 120 magic damage
Now let's get some stats goin onnnnnn for the tanks
: -20% damage from crits, attack speed slow
: grievous wounds, 10% armor +25 thorns
both important effects
full armor tank lookin like: (w/o runes)

With
this puts you at:
4780 hp, 397 armor (80% reduction), 53 MR (35% reduction), with additional 15% blocked per auto AND 20% reduced crits + AS slow, grievous.
Roughly 65 damage in thorns.
Whisper throws 35% reduction into bonus armor, 290 is bonus, means 180.5 bonus for 287 armor. Round up or not, it's w/e. Still well over 70%. After cleaver we'll say about 210, which is around 65%+ reduction.
Bork active will hit for 65, passive for 134 when rounded up.
Duskblade will hit for 112.
Razor for 67 rounded up.
Shiv for 104
Rfc for 78
Now factor in tabis reducing all of that by another fifteen. That total, minus bork active, will be 420 after. Now you lose duskblade, shiv, rfc and you're hitting for just 157 bonus damage per hit, all while taking 65 damage yourself per auto.
You're consistently going to be hitting the guy for 157 bonus damage while receiving a 15% attack speed slow. Maokai's autos will heal him for 686.4 hp when his passive procs and that'll happen very often when you've got abilities hitting you or hitting him. That's more than four of your autos' bonus hp damage dumpstered, at which point you've hit yourself for the equivalent of maybe two.
Let's assume you've got five people doing this damage and for some reason they've all got this armor shredding build so they can deal extra damage to him. Five 420 bursts of 2.1k. He heals back 686 immediately with one auto. This puts him at 3366 hp. Now you're all hitting him at the same time with this total of 785 bonus damage on hit, assuming he's not healing through at least one of those every couple seconds with passive spam.
That's a ton of autos you're all pouring out onto this single Maokai while his backline is firing on you relentlessly. And let me tell you it won't be pretty.
Maokai can throw out enough CC and damage and sustain on his own that he'll lock you down for long enough to make sure his team does the job. Level 18 carry with no boots and maybe you're looking at something around 80-100 armor. Ashe for example has 88 - and if you've got this armor shredding build then your own defenses are basically nonexistent.
Say all of Maokai's team has the suboptimal but same build as the one with 5 AD champs. It's a bullshit build to have but they'll do about double damage per auto as the guys focusing Mao. So yeah, maybe you'll take Maokai down, but in that time you're going to lose at LEAST four carries with the fifth running away from a 1v4.