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Because that is what the measured variance of Major League platoon stats suggests.
Long answer:
There are far more right-handed pitchers than lefties, especially at lower levels of the game, like high school and college. This results in the right-handed hitters who reach the Major Leagues having very little difference between them when it comes to platoon splits. There are two reason for this.
First, batters are selected based upon their ability to hit right-handers. If a guy is strong against lefties but weak against righties, his scouting report would not read "Strong against left-handers; weak against right-handers". Instead, his scouting report would just say "Can't hit".
Second, they are forced to face tons of pitchers who have the advantage over them which means by the time they reach the majors they will have maximized their ability to hit those types of pitchers.
Think about penmanship. There is a big difference between how nice some people can write compared to others. There is also a big difference between how most people write with their strong hand compared to their weak hand. Now imagine if everyone were forced to write with their left hand throughout the entire time they were in school. If after their senior year they were then required to take a test measuring their penmanship with both their right hand and left, what would be the result?
For the natural left-handers, you would likely see a wide range of differences between how they write with their right as opposed to left hand because they had so little practice with their weak hand. So a person with very good penmanship could still be really bad with their right hand while person with sloppy hand writing might be just a little worse with their right hand.
But for the natural righties, you wouldn't see as big a range of differences between right and left because they would have so much practice with their weak hand that they were as good as they could be with that weak hand.
We can extend this analogy further. If colleges required students to have good penmanship with their left-hand to be admitted, what would happen? Number one, it would be far easier for natural lefties to get in. Number two,  natural righties would have a small variance between how they write with their left-hand compared to their right hand because the couldn't get in if they struggled with their weak hand.
This is the way it is in the Majors: lefty batters are far more prevalent than they are in the general population and righty batters have very similar actual (as opposed to measured) platoon splits.