First of all, Fuchsia is not a true microkernel and borrows from both monolithic and microkernel heritage. That is directly the statement from the Fuchsia & Zircon developers themselves.
Second of all, it is BSD-licensed, but it is still open source, ipso facto. Google have even explicitly discussed designing the core operating system around a modular, generic template of sorts such that e.g. Amazon could "plug in" Alexa to the native frameworks for the Assistant functionality. Probably some of this is Google being Google — and being relatively open in their laissez faire attitude toward letting open source engineers do what their heart's desire - and I imagine some of it is about anticompetitive deniability in the case of future antitrust suits (unrelated to Fuchsia, which isn't anticompetitive of them to architect at all despite Nix neckbeard tears, but that's not the point).
Third, the primary goal is to design a new operating system from scratch with a relatively stable ABI and without some of the mistakes of Unix and the NT Kernel — and indeed some of the architecture is laid out in this modularised manner. Moreover, by avoiding GPL & the copyleft licensing that plagues Linux, they knock out a potent barrier to properly upstreaming & releasing OEM drivers.
Wishcasting Linux developers in a tizzy will air out ******** about how "in an ideal world firms would just upstream all their drivers and in doing so open source them" but it's never going to happen at the scale anyone wants, and since the Linux Kernel is refactored without regard for driver interface names used in previous releases, this makes distributing updates independently of a driver's update painful to say the least. There are ways around this, Google's Mainline & Android GKI (mandatory in A12) has been dedicated to doing so, but the success has clearly been limited and thus it's very obvious why Google would want to start anew sans legacies.
There is a lot more truth to the idea that iOS and MacOS are bloated than I think most want to acknowldge. Windows on equivalent hardware really does run smoother. Just take an intel MacBook or even similar generation i5 laptop vs a MacBook with an i5 - even with Iris graphics, Mac OS just chugs, and even on an M1 it feels that way.
It's been like this for years, since Windows 8/10 in terms of performance. The fisher price eye-candy GPU shading and and small army of MacOS daemons and irrevocably installed first-party applications or memory/kernel panics are just absurd. Windows is just lighter, Windows defender aside.
Years were spent talking up storms about ill-defined optimization techniques with regard to Apple's products. While it was true Apple's iOS displayed frame tracking and touch latency response was superior to Android and really still is, Windows Phone proved this wasn't necessarily about vertical integration - the touch response latency was similar to Android but the actual frames animated on response to scrolling felt much more natural.
Anyways, overlooked was and remains the *benefit* Microsoft and Google's teams have in that there exists pressure toward designing with lowest-common denominators in mind - and in fact as of Android 12 and Windows 11 as opposed to Monterey and iOS 15, it's frankly alarming how obvious it is.