Why would that be a desirable quality? Wifi devices (using Matter or not) live on the same network as my PC - meaning a compromised lightbulb (or one that hasn't been updated) can be used to infiltrate and attack my home computers.

Thread+ Matter, despite using a different radio, suffers from the same issue, since a border router is on the Wifi network, a smart bulb using Thread can theoretically access my PC.

Yes, I'm sure there are ways to fix this, but why have the problem in the first place?

Zigbee is entirely incompatible networking standard, and doesn't have this problem.