The problem is simple.
A lot of companies end up being run solely for the benefit of the shareholders (often pursuing short-term returns) than their customers, or even the long-term interests of business itself.
This is causing not only businesses to make terrible anti-consumer decisions, but is also causing products and services to visibly get worse over the years, due to cost-cutting, etc. It can and does lead to staff being treated poorly.
There is no doubt that shareholders end up being the most (only) important thing. More important than the business; the product; the staff.. everything.
You can pretend that shareholders are win/win for everyone, but reality is they often behave like a cancer the business can't cut out. The pursuit of shareholder returns becomes the sole focus of the business.