AWS has an enormous first mover advantage. There were many well-respected pundits who thought Azure was going to completely flop-- Microsoft had no previous experience building anything like it, and AWS was already a rich ecosystem. (I worked at Microsoft when Azure was launched, and I thought it was going to flop, too, but I'm not a well-respected pundit.)
The fact that Azure is within an order of magnitude of AWS is a minor miracle. If revenue is 1/2 of AWS, that's an astonishing success. (And looking just in terms of raw dollar figures, it's hugely profitable for Microsoft, regardless of how it stacks up to competitors.) And while it's true that some portion of that comes from hosting Office 365, it's also true that AWS runs all of Amazon's infrastructure, and they effectively act as a logistics company for a huge number of other companies.
My company is multi-cloud, with substantial infrastructure deployed in GCP, AWS, and Azure. Years ago we were originally just AWS, but (at least according to company lore) our CEO got frustrated that Amazon wasn't willing to give us better pricing-- basically, he felt that we were big/important enough that we should be getting more preferred discounts.* He went out and signed a deal with Google, without (IMHO) understanding the enormous cost in employee hours that it would take to switch.** Then, most recently, our largest business unit has switched to Azure, because of pricing and apparently (and I'm not in-the-loop or an expert here) also because of some of their ML/AI capabilities.
People that come from exclusively AWS shops usually start off frustrated with the limitations of GCP and Azure, but quite honestly, six months later they're usually getting work done almost as efficiently in those (significantly cheaper) options. I've been reviewing a lot of resumes and screening a lot of applicants for some openings on our Security Operations team, and while AWS is the most common cloud provider people claim familiarity or expertise in, it's not that uncommon to find someone who is passionate about Azure. I don't recall finding someone who was particularly attached to GCP. If I were to judge by the candidates I'm seeing alone I would probably have guessed market share and/or revenue at something like AWS 50%, Microsoft 35%, and GCP 15%. I realize this is anecdotal and based on memory, so I'm not suggesting it as a serious data point, just sharing my personal experience.
* - I want to stress that I have no personal knowledge of what happened, but this is the commonly-told explanation within the company.
** - On the other hand, regardless of how the decision to switch to GCP was made, I am firmly convinced that it was not a profitable decision, due to the overall cost to the company of switching-- not directly in payments to the cloud provider, but in hours worked, consultants hired, work on profitable projects being delayed by six months or a year, needing to integrate systems running on GCP with those that could not, for technical reasons, be moved off of AWS, more expensive administration/maintenance because of the relative immaturity of the product (particularly when we first made the switch, Google has added a lot of functionality since then), etc.
