The idea is that you can't, on average and long term, beat the market. So paying extra money for a fund that claims to be able to do that is an unnecessary gamble. Accumulating the expertise to evaluate a fund's ability to perform better than the market would give you the ability to just invest at that level anyway, so you might as well save your time and money and stick it in the cheapest market funds you can manage.

Yes, some strategies beat the market, sometimes (they also sometimes fail catastrophically). But you can do pretty damn well comparably in the long term by having a very low-cost, low-effort strategy that frees up a lot of time and effort for other pursuits.

You can look up expense ratios on Google, Morningstar, etc. Vanguard does pretty well. They're pretty well represented here.