It is my experience, that Outlook is not a reliable e-mail service. Sometimes e-mails are not delivered, or only delivered hours later. When they are delivered, even as a paying customer, they are downloaded so slowly, that I had to wait 10 minutes to get all my e-mails, while my 1 EUR per month Posteo provider delivers in seconds.

My impression is, that the only reason one would want to have MS as a mail provider is, that they are entrenched in the e-mail provider reputation and delivery game. Other than that, it seems to be an all around bad service. Not even talking about the mail client itself.

The big reason is enterprises buy into O365 and running their email through Outlook instead of on-prem or at another provider is part of that. For the same reason they use Teams over Zoom or Slack or other alternatives.