This is actually addressed on the BDS movement website

https://bdsmovement.net/what-bds

Effective boycotts work on companies that are large enough to be noticed but small enough for boycotts to have an effect, while divestments and sanctions can be lobbied for larger institutions and governments.

I think just saying something like "why not just boycott all huge corporations/organisations?" can be a bit disingenuous when I think it's pretty well acknowledged that We Live In A Society that makes completely separating yourself from large corporations essentially isolate you from society as a whole