Easy - because people thought they improved performance, and so it was banned. Nobody thought to test, back when they were first banned, because it was just universally agreed that it worked, common knowledge and all that.
Nowdays, the rules have been changed from those, and they’re illegal not because they’re specifically corked bats, but because any bats modified with a foreign substance are illegal. You’d get the same penalty if you filled it with a steel bar, or lead shot, or runny custard.
(though I desperately want to see someone in the major leagues break a bat, and custard just starts pouring out. Scott wants more exciting sports, I just want more absurd sports.)
And before you say “But pine tar is a foreign substance” - there’s a bunch of rules regarding pine tar, including where and how much you can put on the bat, and what’s approved for use.
If they worked as suggested - which they don’t - then yeah, it would be something similar. But the issue at that point is safety - you’re firing a ball out at a potentially lethal speed, faster than the pitcher, who wears no protection, can realistically react and either duck or otherwise try to protect himself. And unlike corked bats back in the day, that one was actually tested - they simply view it as an unacceptable risk for the batter strength you tend to see in the major leagues.
Edit - Random fact, they tried aluminum Cricket bats too, which are now also banned - but not because of saftey reasons, it’s because they damage the ball.