I don't think it is the only reason. Labels in Gmail are still really nice. Microsoft Outlook rules will often duplicate messages and smart folders using search

I don't think it is the only reason. Labels in Gmail are still really nice. Microsoft Outlook rules will often duplicate messages and smart folders using search filters don't work between browser and email client. Using IMAP they are simply virtual directories like they should be. Microsoft Calendar is also pretty bad too. Like in a large organization you'll often have people leave the company. When you delete a series in Outlook with multiple people it tries removing the history for everyone which is important dates. There are a lot of nuances that would prevent me from ever switching to Office. Even OneDrive is tied to SharePoint and it is way too easy to break.