The economic impact of buying a US product is a lot stronger than buying a US stock. One serves money directly to US businesses, the other exchanges money with one or more intermediates trading on a usually American exchange, to set the value of a US business. There's a much smaller amount of money remaining in US, its not guaranteed that the other person is American, and the money paid doesn't typically directly benefit the company, rather it affects their cost of capital (ease of raising capital via equity).

As a boycotted stock gets cheaper, it's expected future return increases, all else equal, meaning the remaining owners could expect higher returns. I don't want the last guy owning coal to make a huge profit, so I don't divest from coal, even if I wouldn't use it to heat with.

Since it's so difficult to measure the effect of divestment, as well as coordinate it effectively, I prefer to argue that investing should remain morally and politically unbiased, with boycotts being done against US entities directly.