Not standardising means being able to distinguish yourself easier, it's just that for anything that integrates with third-party stuff, standardisation is way cheaper. Apple doesn't make monitors or flash drives, so making their own specs for those wouldn't be beneficial to them and would increase prices of compatible products. But a filesystem is something they do make and being able to do whatever they want with it is quite beneficial, with no downsides as there are no third parties (that they care about) integrating with it that would have to put in extra work to support it (besides their direct competitors, which is a nice bonus).