They don't need to. There is a great many of more cost-effective ways of causing havoc, and even if they really wanted to use GPS-guided missiles, it's probably a better bet to build slower missiles with consumer hardware, but build a lot of them - missile interceptors are very expensive. If a DIY $1k missile needs to be shot down by $1M Patriot, they'll be doing lots of damage with a couple of them, without even hitting anything.

Yes. Look at the ham hobby, hardly any of them can pick up GPS signals, despite the technology being literally everywhere

That's not true at all. You can use a $20 SDR card to receive GPS signals and and then can decode them on any commodity laptop, or a small computer like a raspberry pi.

e.g. https://www.rtl-sdr.com/receiving-gps-with-an-rtl-sdr-dongle...

Receiving the signals is only a part of the problem, and even getting a "fix" isn't all that hard. Maintaining an accurate fix, and at the same time refining away previous errors, on a fast moving vehicle, is a dark art.

Software architectures are the limiting factor here.

Every app on your phone cares about "Where am I now", but no app wants to know "Where was I 5 seconds ago, but with more accuracy than you knew when I last asked".

Neither android not iPhone have an API to allow GPS hardware to refine the accuracy of historic position locations.

Possible maybe but unusual. 99% of ham is narrowband, GPS is spread across a megahertz. Somebody goofed and made a TV tuner chip that could be used as an SDR at that sample rate, and I'm sure the powers that be arent happy.

You what?

Any sensitive Spectrum Analyser or SDR can see the bump in the spectrum caused by the GPS Signals. And quite a few amateurs have built homebrew GPS receivers.

Or they can buy a little GPS module and use the data stream for a huge range of projects.

Most modern digital Ham communication methods require a 10MHz feed from a GPS module to maintain sufficient time and frequency accuracy.