Why can an uncoordinated call for all companies to leave undermine legitimate state sanctions?

Because states deliberately keep essential goods off their sanction lists to spare civilians; a general exodus turns targeted sanctions into immoral comprehensive ones, with unpredictable humanitarian consequences, and escalates sanctions beyond the control of the sanctioning states.

The argument runs in steps:

1. Moral sanctions must be targeted. A general call for all companies to leave amounts to immoral comprehensive sanctions.
2. Stopping the supply of vital goods harms civilians, which violates the principle of discrimination. So companies must act in line with the moral sanctions and keep supplying essential goods.
3. Sanctions regimes involve many actors, so even their designers cannot fully predict the economic and humanitarian effects. Unpredictable economic consequences mean unpredictable humanitarian consequences, which is why states widen sanctions cautiously and step by step.
4. States face a real dilemma. Broader sanctions promise a faster end to the victims' suffering, but become less predictable and hit civilians and third countries.
5. If companies leave en masse, sanctions escalate without the control of the states that are supposed to decide them, which counteracts the states' intentions.

Companies selling non-essential goods may still feel the pull of the extended complicity argument (the tax argument) and choose to leave. That is their decision, not a moral obligation.