By contributing to demand for a stock you increase the valuation of that stock. Securities Based Lending is often how companies and executives secure loans and avoid taxable events. By contributing to demand for a stock we facilitate additional funding for the issuer of the stock and it's largest shareholders.

I absolutely agree, cash flow is a much more immediate concern to any company, but one wealthy shareholder divesting can have the same financial impact as ten thousand average citizens boycotting. Local investing is more difficult and risky, but also more rewarding and necessary. It is not just about a monetary return, it is about building social capital and local resiliency.

You're arguing that people should give no consideration to the long-term social and ecological harms of their investments beyond what will make them the most money. By directing our actions in that purely incentivized way we sacrifice everything unprofitable, and that alienation is exactly what causes so many chronic societal issues. I agree that an individual can have very little impact alone, but capitalism places this burden at the individual level.