M365 is used only because it is the continuation of MS Office, which had been entrenched in most companies for many decades.

In the beginning, the fact that MS Office was proprietary had nothing to do with communication protocols, but only with the file formats.

The need to convert between proprietary file formats had always existed in an enterprise setting, which is why all such products, including MS Office, had extensive support for importing the file formats of their competitors, so this was never a serious obstacle for adoption.

It’s all that survived.