>Why wouldn't a person build that into the heart of something important?
Because it's inexpensive and not designed to be tamper-resistant. If preventing this type of thing is your goal there are chips out there designed to break irrepairably if tampered with.
Rp2350s are advertised as having quite a few anti-tamper functions. They had a bounty when it launched to find similar vulnerabilities and they worked to patch the ones that were found. This is a lot more credible than a lot of advertised anti-tamper features.