Some people just don't like it because they don't think it's fair.
Imagine you're an executive with inside information that you think will make the stock go up. So you buy some stock (from an existing shareholder, obviously), and then the stock goes up. That shareholder rightly will feel stiffed.
Conversely if you sell stock and it goes down, the person you sold it to--now a shareholder to whom you have a fiduciary responsibility--is left holding the bag.
The fact that it's an insider rather than an outsider that they're trading with doesn't make any difference to whether they win or lose.
The only fair system I can imagine (fairer than the status quo by my estimation) would be complete transparency updated on a continual basis. But I can imagine it would be difficult to keep that up while making productive business relationships.
Matt Levine has written about this many times, here's one:
https://www.bloomberg.com/view/articles/2016-12-06/supreme-c...