In a small business, maybe you're the only person doing <critical task X>. This is a more level playing field, but maybe a union is still useful here also.
Why do you think employees have enough leverage across most SMBs to the extent that workers shouldn't organize?
Even SMB owners and managers organize adversarially against their workers (eg by setting compensation and benefits per VC or consultant provided guidance on labor market, by hiring HR specialists to coordinate with managers on keeping wages as low as the market will bear). Why should workers not in turn organize in those cases?