No way BAB could ever be equal to full casting, but it wouldn't compare quite so badly if not for two main problems: What it does isn't exclusive, and it's not the best option for what it does. All BAB is good for is certain opposed combat maneuvers, killing people with damage, and granting extra attacks. High ability scores and big size help with 2/3 of that, all 3 if you count Combat Reflexes as extra attacks. Worse, they do it far better than BAB does. Natural weapons blow BAB out of the water in terms of getting extra attacks, even without Combat Reflexes. A dire lion, a mere CR 5, gets more attacks on a charge (claw x2, bite, rake x2) than anyone can ever have from base attack bonus, and it gets them before BAB has even granted its second attack. Better yet for the lion, all but one of those attacks is at full attack bonus, and the trailing attack is only down by 5.
For BAB to really shine (though still nowhere near as much as full casting), it would need to be the first and final word in combat maneuvers, damage output, and number of attacks. A barbarian losing a grapple with a level-appropriate undead (2/4 BAB) should not even be a possibility to consider, even if it is 3 sizes larger and has a Str score in the 40s. A wizard's best option for solving a problem that needs to die should be hiring on a knight or the like and pointing him at the problem (none of this heavily metamagic scorching ray, or save or die spell type of stuff). The fighter who TWFs should be making at least half the attacks per round (counting both sides) in any given combat that doesn't include another 4/4 BAB class, with most of them at or near his full attack bonus.