If the open source implementation is equally good, I'm sure people will use that instead of Tailscale. That Tailscale exists makes me suspect that the open source implementation - as is usually the case with these "just use curlftpfs!" comments –
is not equally good.
The reality is that making software, like any other human endeavour, takes time and energy. Paying one another money is a rather well-established mechanism of rewarding and incentivising that time and energy (since not everyone wants to work free of charge to make and maintain software for you, out of the goodness of their hearts, no matter how much you insist that you're owed their unpaid labour).
There are small and local means of getting free food, or free woodworking, etc, but the general reality is that a high-quality high-dependency maintained product, over the long term, is more feasible when it's paid.