It's not in the interests of long-term investors to keep advantaged traders out of the market. The alternative to the participation of informed investors is mispricing. Fundamentals investors want prices to reflect actual value, too.
Isn't this what insider trading laws try to do? The law obviously can't keep insiders from learning information first, so the law prohibits insiders from acting as investors until the info is public. At worst this creates a lag between information and price, but that wouldn't matter much to long-term investors.
Not that insider trading laws can create total information awareness for every investor. But at least it gives every investor a more equal opportunity to find and use information.
minor edit for clarity
Anyone with the time and means to do so, anyway. Anyone could do these things; but most don't. For the people who do, what is protected is their investment in gathering and analyzing the data. The law does not grant Person A access to the data set of Person B--true--but the question is whether Person B could do their own work to gather the same data set.
Whereas information that is only available, in any form, to Person A would be considered non-public, and not legal to trade on.
If all traders were equally informed (perhaps by a law that any information you wanted to trade on had to be public), there would be no way for people to profit by digging up new information. We (society) like it that analysts have a financial incentive to figure out the truth about how well a company is doing - whether that be by coming up with a better model of how one industry affects another (which will ultimately lead to better allocation of resources), or doing the legwork to realise that a particular company is a massive fraud (which leads in a more direct/obvious way to better allocation of resources).
But we don't want them to just bribe insiders - that causes agent-principal problems, gives certain market participants unfair advantages, and all the rest of it. Hence the law, where figuring out these things through research is encouraged, but getting them from insiders is illegal.