Because outside of a handful of nerdy tech companies, all small businesses need to use Microsoft Office. From there, it’s a no brainer to stay in the MS ecosystem and use Sharepoint etc…

For a small business without a dedicated IT team, simply hire a IT contractor to harden the tenant (MFA etc…), have them review every six months and be done with it and focus your resources on running your business.

My father’s decidedly non-nerdy logistics consulting business with roughly 20 employees ran (and runs) on Mac OS since the founding of the company in the mid 1990s with my mom being the „IT team“. There are some situations where companies rely on certain compatibilities requiring windows. But most could do completely fine without, especially nowadays.