a) VirtualBox is an oracle product. That by itself should be telling.
b) High sierra is unsupported as a Guest OS in VirtualBox. You do know what that means, right?
c) You seriously complain about the darth of open source virtualization for an OS, which disallows virtualization on anything but apple hardware? ...really?
If you want good open source virtualization you'll want to use Qemu/KVM. Which obviously doesn't support any apple OS either, because they're not allowed to virtualize it. Take that up with Apple, not open source
Putting aside the fact that this isn't true, and that there are quite a few quality open source apps on macOS, it's pretty clear why paid products have higher quality: the bar for people to buy them is much higher, so they generally need to be at least somewhat decent for people to consider paying for them.
So, virtualization is technically only allowed if you're running your apple hardware with anything besides Mac OS.
But apple isn't enforcing that limitation, as products like VMware fusion on Mac OS are (at least as far as I can tell) officially sanctioned.
If you obtained a license for the Apple Software from the Mac App Store or through an automatic download, then subject to the terms and conditions of this License and as permitted by the Services and Content Usage Rules set forth in the Apple Media Services Terms and Conditions (https://www.apple.com/legal/internet-services/itunes/) (“Usage Rules”), you are granted a limited, non-transferable, non-exclusive license:
(i) to download, install, use and run for personal, non-commercial use, one (1) copy of the Apple Software directly on each Apple-branded computer running macOS High Sierra, macOS Sierra, OS X El Capitan, OS X Yosemite, OS X Mavericks, OS X Mountain Lion or OS X Lion (“Mac Computer”) that you own or control;
(ii) If you are a commercial enterprise or educational institution, to download, install, use and run one (1) copy of the Apple Software for use either: (a) by a single individual on each of the Mac Computer(s) that you own or control, or (b) by multiple individuals on a single shared Mac Computer that you own or control. For example, a single employee may use the Apple Software on both the employee’s desktop Mac Computer and laptop Mac Computer, or multiple students may serially use the Apple Software on a single Mac Computer located at a resource center or library; and
(iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software, for purposes of: (a) software development; (b) testing during software development; (c) using macOS Server; or (d) personal, non-commercial use.