So, same reason why Microsoft uses SAP instead of Microsoft Dynamics. They would use their own product if they were to start now, but migrating from SAP would cost them millions and take a lot of time.
An example is 3D printers. Prusa makes fantastic 3D printers, and about half of the structural parts for their printers are printed on their own printers. So they have 600 of their own 3D printers printing 24/7 printing their own parts. That means they are forced to address long-term durability and reliability issues even just to ship their own product.
Because it absolutely is true that at that scale, you're usually better off just injection molding the parts, but they'd lose the high-quality signaling that yeah, their 3D printers are good enough for the company to rely on them to print their product. They also lose out on insight to things they could do to improve their own product from a usability standpoint.
Microsoft's annual revenue is almost US$200 billion. Even if the migration would "cost them millions", they can easily afford it, it would not make any material difference to their ultimate financial position. And, it would actually be justifiable as an investment in their own ERP suite – "eating their own dogfood" would send a signal to their customers about their commitment to those products, and would also help improve those products over time (by exposing them to the distinctive needs of a Microsoft-scale enterprise). As far as time goes, they could always do it gradually (maybe they already are), and even if it takes several years, before we know it those years will be behind us.
Of course, Oracle has so many different software offerings (duplication due to acquisitions, and lots of industry-specific offerings), it couldn't possibly use them all internally. But at least its sales reps could say, that if they weren't using the specific product they were selling, they'd be (for non-industry specific functions) using an equivalent product in Oracle's portfolio.
Amazon themselves say it is true: https://aws.amazon.com/blogs/aws/migration-complete-amazons-...
Although note this small print:
> Amazon’s Consumer business just turned off its final Oracle database (some third-party applications are tightly bound to Oracle and were not migrated).
No idea what those “third-party applications” are and how significant they are - they could be insignificant fringes or extremely core business systems (such as payroll or general ledger)
Probably they've migrated off now, but IIRC they've used PeopleSoft, because of course it is.
So, Microsoft's own Dynamics offers no greater value than their competitor SAP, even within ... Microsoft.
[0] Recent example - https://www.oracle.com/cloud/oracle-at-oracle/ - although they were saying similar things 15 years ago, albeit minus the cloud part
The departments are also not the same, the developers might be happy to have a customer like Microsoft but the hundreds of people using something else today really don't gain anything. They would have to retrain and take on a huge migration. And for a system that has grown for such a long time you can be sure that there are a lot of undocumented edge cases for weird business needs.
Smaller companies have burned millions trying to move off SAP. And there is nothing Microsoft can do or learn to make the migration to their product easier because it's all business problems, not so much technology. Their potential future customers aren't billion dollar companies which are on SAP today.
FWIW, we're happy Stripe customers, but also candidly do much of our billing outside of of them. Some of the examples GP cited are in a similar boat. Which is all to say that Stripe solves a lot of headaches, but it hasn't solved all of them. I'm sure investors in Stripe see that as a positive opportunity.