Megacorps weren't always giants and it's not unusual for small companies to eventually become giants through excellent vertically-integrated products, and such companies would become subject to these regulations.

Interoperability is not free. One of the trades it brings is a notably lowered ceiling in terms of tightness and capabilities, and this persists no matter how many man-hours are poured into engineering the systems that enable it.

The Linux desktop is a great example of this at play. While it's technically worked for decades at this point, it's been a constant struggle to make it a high quality, thoroughly polished experience end to end and that's partly thanks to the unavoidable friction and gaps between layers that comes with interoperability and tens of involved parties.