Vendor lineage
Arm - the processor almost nobody buys from Arm
Formed 1990; licenses a processor architecture rather than selling processors.
Arm was formed in 1990 as a joint venture and sells something unusual: not chips, but the right to build them. Companies license its processor designs, add their own logic, and have somebody else fabricate the result.
The consequence is visible in almost every device this site describes. The management processor inside a switch, the controller on a network adapter, the data processing unit doing encryption on a card, the access point on the ceiling - a large share of them run an Arm core that the equipment vendor licensed rather than bought.
It is the third separation in the arrangement the making milestones record. Manufacturing had already been separated from design by the foundry; the architecture was then separated from both. A vendor can license a core, add its own packet-processing logic, have a foundry fabricate it and a contract manufacturer assemble the product, and own none of those four things.
That model also explains why the architecture spread as it did. A company selling chips competes with its customers when they want to build their own; a company selling licences does not, so the people best placed to design specialised silicon had no reason to look elsewhere.