Send all non-contact calls to voicemail

Why doesn't Apple (or Android for that matter) allow us to send all non-contact calls straight to voicemail?

I'm sure they've thought about this, and given the spam call epidemic it seems like an easy temporary fix to the problem until a more systemic one can be reached. I'm wondering if there's some legal or contractual reason that prevents them from taking this approach.

Background

I've been receiving several spam calls several times per day for 1-2 years now and the frequency of calls has been rising. I've read enough news articles to know this is a widespread issue. I've considered using Do Not Disturb mode but that would also block notifications I want to receive. I've used spam blockers like Hiya and RoboKiller to stop the flood of spam calls, and while they do work to some extent they aren't perfect. A simpler solution would work even better (for me at least):

Automatically send all calls to voicemail unless they are in my contacts.

This seems like a straightforward and effective way to combat the distraction caused by spam calls. While it would be somewhat annoying to get my voicemail box filled with spam, it's better than having my phone ring/buzz.

However, Apple doesn't provide a way to approve numbers. Rather, they only let you block specific numbers. Since spam callers are using services that allow them to obtain new phone numbers easily, block lists are easily circumvented. An approve list, however, would not be easily circumvented since it is difficult (impossible?) for a spammer to use someone else's number.