But DHCPv6 also supplies INFORMATION REQUEST which designed to operate without address assignment. It supplies things like DNS.
With Windows not supporting RDNSS in RA, it requires DHCPv6 DNS via the information request to work in a pure IPv6 world.
In the same vein, Android not supporting DHCPv6 requires RDNSS in the RA to work in a pure IPv6 world.
So as it stands right now, we have to run both if we want to support both OS and that sadly means redundant data going around the network.
I cannot speak for Microsoft, but I do know that Android uses an old version of dhcpcd.
dhcpcd has supported DHCPv6 for a long time now, so they just have to upgrade it to get it to work, which means this is purely a political rather than technical problem.