There's a <50% chance you'll make money off of it because it's zero-sum, and if insiders make money on average then other traders (i.e., you) have to lose money on average.

The issue is that the odds aren't actually 50/50 on you buying either side of the trade; one half will look like a better deal (and given public information, it is a better deal) so you'll buy that half. Then when the market resolves, it'll turn out that insiders knew some piece of information that made the other half of the trade a better choice.