Significant stock ownership is pretty public through 8k forms. The SEC doesn't require reporting every trade of a few million dollars because it generally doesn't make a difference anyway. And you CAN access time and sales data in most brokerage platforms to see the size of many of those trades. The big exception there being those done on dark pools.
And while perhaps you could argue against dark pools, most would agree that illiquidity is a bad thing in markets. Dark pools do a good job of addressing this allowing for large positions to be massaged into a market they'd otherwise disrupt fairly heavily.
Funny how often we allow shitty practices in one area because we already allow shittier practices in another. Why not have good policies instead of accepting bullshit all the time?