Customers because a product they use from a company won't work because...?
Employees because they not only work at the company (I'm assuming they're paid for their labor), but also can buy stock in it?
Taxpayers because of bailouts?
When you have a small business, or even a large business that has a single owner, that person has to care about things like their own reputation and might sometimes have preferences for things other than maximizing returns. There is a significant difference in behavior between a profitable company that makes money by providing something people want and a company that makes slightly more money by turning the screws at every opportunity.
And typical funds only care about number go up. Even the ones that nominally exclude "bad" companies/industries have their rules, but then hold shares in Comcast rather than Exxon and still expect Comcast to turn the screws. Whereas to do the real thing, you would need funds with missions to do things like specifically fund companies who are new entrants to concentrated markets to increase the amount of competition. Which to most funds is a conflict of interest, because if the fund has that industry as a target then it already owns shares in the companies the new entrant would be competing with. Conversely, mergers they're all for, because they own shares in all three companies in that industry, so causing three to become two is higher margins for everybody.
So it produces market concentration and drives obsession with quarterly profits, harming customers and employees. The consequently larger companies then buy off the government to get subsidies and insist they're too big to fail whenever they screw up, use the nature of multinational conglomerates to arbitrage tax rates and shift tax incidence to purely domestic companies and workers, etc. All of which the funds like because they make number go up.
It's basically a paperclip optimizer which is trying to accumulate money rather than paperclips.
Maybe? Maybe not! Just saying it doesn't make it true.
I'll skip the step asking Claude to produce the lengthy list of examples where market concentration richly benefitted customers, employees, and taxpayers.