WiX is great. I have been using it for over a decade to package an internal application. It's all driven by a simple text file which I liked over Microsoft's packaging offerings.
It's not. It's an open source project (aka owned by no one) that started at Microsoft. The person who created WiX left Microsoft to create Firegiant, which is the primary contributor to its continued development and maintenance.
Open source does not mean "owned by no one". It means the owner or owners have licenced their intellectual property to you under the terms of an oss license.
I only know about other proprietary non-MS solutions like InstallShield, which also generate .msi files but use their own proprietary way to define the installation process, with the benefit/disadvantage (depending on what you want to do specifically) of being on a higher abstraction level than WiX, which is more or less a direct resemblance of the internal Windows Installer data structures.