mysql-server doesn't belong to MariaDB, they can't "own" it. Doesn't Oracle hold a trademark on it?
Whereas Windows apps tend to standalone, bringing with them any libraries they need, Debian tends to have one copy of each library, runtime, and so on. If you upgrade Debian, you upgrade way more than just the kernel, the gnu tools etc. You also upgrade, for example, the Python, the windowing system, the networking stack. Apps that rely on these must also be upgraded in order to work with the new versions of whatever other packages they rely on.
It’s a domino effect and it keeps on knocking right up to GIMP or, in this case, MySQL.
My answer doesn’t explain why this particular choice was made but I hope it shows how Debian can do that sort of thing whereas Windows wouldn’t.
I guess the fact that mariadb now owns the "mysql-server" package name is a side-effect of how apt manages upgrades and migrations. It's probably the best way to ensure existing users end up with a supported mysql-ish server after the upgrade, instead of being silently stranded on an obsolete debian8 package without receiving security updates.