Many apps (e.g. EteSync and WireGuard) are almost useless if they don't work for everyone within a certain group. A more extreme example is a messaging app. Will not having iOS support for a messaging app lose you 40% of your users? No, it will lose you 100%.
In WireGuard's case it's maybe less obvious than messaging, but if WireGuard doesn't work on macOS, it's enough to have one Apple user in your whole organisation in order to make it a non-viable solution.
> So if bigcorp wants OS X WireGuard support, they should be able to pay handsomely for it.
Who says they're not? A lot of the companies on https://www.wireguard.com/donations/ ship their own macOS software. Just because the Wireguard Mac app is free doesn't mean nobody's giving them money that's earmarked for Apple development.
Video editors, designers, and sound mixer are a few example professions where users mostly use Apple products. Most companies have designers.
Additionally, companies don't choose their whole software stack based on their VPN solution. They would just change a VPN solution if it's incompatible with what's there.
By now, video editors and sound mixers are heavy windows users, because there's no halfway endurable Apple machine that you can purchase that supports 128GB of RAM and 8+ CPU cores and NVIDIA CUDA. Because like it or not, almost all video editing plugins use CUDA for acceleration.
In WireGuard's case it's maybe less obvious than messaging, but if WireGuard doesn't work on macOS, it's enough to have one Apple user in your whole organisation in order to make it a non-viable solution.