EDIT: To clarify, I am surprised that Riot allows this when they have the "build no damage, do no damage" rule going on.
Bruisers are kind of a special case.
Assassins, mages, and marksman are supposed to build damage to deal damage. Melee-carry-esque fighters as well.
In all those cases, the champions actually come with pretty impressive defensive mechanisms built into their kit though. Like range and mobility, and on the melee carry side things like Endless Rage, Wind Wall, and Diplomatic Immunity that just don't scale with defense to any capacity.
As for tanks and supports - those often remain useful with pretty powerful utility effects. Mostly CC, which makes those as big of a problem as all the damage on their team, despite just going heavy tank/utility builds.
However - bruisers arn't particularly mobile, they don't have much in the way of range, and they often don't have much in the way of crowd-control.
Bruisers are tanky DPS. They rely on being tanky to deal DPS. Unlike marksman, assassins, and mages though - they often come with very little in regards to survival mechanics.
They have to build tanky to function.
Which causes a problem concerning the DPS part. If you force bruisers to itemize for damage to deal damage, but you also force them to itemize for defense to have defense - they'll just end up dealing mediocre damage and having a mediocre level of health.
Like everyone esle that does that - except what little defense bruisers come with comes noware close to the level of protection range, mobility, and even CC provide for just about everyone else.
So, ultimately, it seems Riot decided to give Bruisers a boatload of damage that always allows them to be DPS while forcing them to build tank. Mundo isn't a special case among bruisers concerning stupid amazing damage either - Nasus, Shyvana, Warwick, Darius, Udyr, Renekton, they all deal a pretty impressive level of damage without actually building any damage.
Which in turn lets them build full-tank and function the way they're supposed to.
I should also note - bruiser damage tends to have awful scaling. A lot of the things that make it really solid without offense also cause it to be kind of mediocre with offense. Most bruisers are split pretty evenly between AD and AP, auto-attacks and abilities, physical and magical damage. Which really limits there damage scaling. So - they have issues using offense, but they scale super-well with defense.