Likely for the same reason everyone tries making their own web browser, even though the other guys' are all gratis (and many of them libre): When something is a platform, you are either a landlord or a tenant.

Google and Apple have seen what happens to Microsoft's tenants, so they decided to be landlords. Microsoft knows how awful a landlord it had been (and after decades of landlord-only status, suffered abuse as a tenant at Google's gmail and youtube platforms), so it also tries to be a landlord in every way it can; It couldn't attract it's own tenants to Windows Phone, IE11 and its own Edge, so it has to offer subleases on Android, iOS and Blink(=Edgium) these days.

QUIC looks more "behind the scenes" as a platform right now, but building your own is a very cheap hedge against ceding complete control of what could become a potentially fundamental platform to your competitors. So everyone does that.

I'm no fan of Microsoft, and I believe that Microsoft has been "beaten to submission" rather than "left the dark side", so to speak. But regardless of the overall technical quality or moral/values one assigns to Microsoft - they are a smart, politically and business oriented and savvy corporation. This is a "staying relevant and in control" move.

Possibly the result of an industry scarred by the experience of depending on a TLS library that everyone thought was secure because “hey everyone is using it”. Now they want to make sure they understand and control critical security infrastructure.

Multiple implementations of a protocol are by no means a bad thing. The opposite is a problem - too few implementations means that the implementations dictate the spec and creating something compatible requires implementing bugs of those implementations too.

That said, I’d be surprised if we still had this many implementations 5 years from now. I think at least some of those will become unmaintained and instead use what Google/Microsoft/Mozilla/Cloudflare have developed.