No organisation of any size buys everything from one vendor though. Microsoft dominates desktops, but Apple and Google dominate mobile devices, an organisation might have Oracle databases running on Linux servers on top of that, some SaaS suppliers, some desktop software suppliers....
But still - think how much more you'd need to buy and validate it all works together. Microsoft gives you AD, which works with Outlook, Sharepoint, Azure, Office365, Teams, then all of those, plus Excel, Word, Powerpoint all bundled, and not for very much money.
True, but it varies depending on how well those fit your organisation's needs whether buying into the full bundle is what you want to do. The less of it you want, the less the advantage.
Most people using MS desktops use AWS rather than Azure. Lots of software from other vendors does reliably work on Windows.
