There are a series of game related companies, that build products for other games companies, such as (a small taste, plenty more then listed below)
Also depending on how you define platform, there are companies that provide backoffice tools to build your company, like SAP, Oracle, IBM, Lotus (in addition to Microsoft/Apple)
There are also hosting platforms for code like
I guess it comes down to what exactly you mean by platform. The phrase is too general, and I've been limiting my answer to simply software platforms (let alone say electronics hardware development platforms, or manufacturing platforms, or trading platforms).
There are plenty of examples in pretty much any business or academic focused area that has vendors, both startup and established, providing significant ground work for other groups to achieve their goals. In addition, their profitability and growth is wildly variable consider the scope of groups that "platform" can cover.
Edit: One more, you are most likely aware of Salesforce. Even if they don't like to admit it, they are a software platform.