See https://helpcenter.steinberg.de/hc/en-us/articles/115000177084-VST-plug-in-locations-on-Windows
VST3 plugins are executable program code, and as a subdirectory of C:\Program Files\Common Files is where the Microsoft Windows guidelines specify to put (executable) components that are shared between multiple applications, arguably that is where the VST3 plugins themselves should go. The way VST2 handled this was a total mess IMO and I’m glad it’s been fixed for VST3.
.vst3 files are generally quite small, most of mine are in the region of 50MB or so, and much smaller than the sound libraries that come with them, so I really don’t think leaving them on C: is going to cause any bother for most people, unless you have a C: drive that is massively too small for your needs. Given that most decent plugins allow you to store the sound libraries themselves in a different location.
Many (mainly older) applications won’t even let you install programs to a drive that isn’t C: as they can’t cope with this scenario.