Some folks take it as a challenge to replicate or "improve" another's design. If you've invested months in a piece of work, done on speculation, you don't want someone else to be able to copy it and deprive you of compensation for your effort. Quite frankly, if the crystal is attached where it is attached, it's unlikely they'll fail to recognize the part if they're in any sense, sophisticated. Combine that with the telltale reset circuit, and the crystal frequency, and the cat is out of the bag, so to speak.

Back when we used lots of Schottky TTL, boards got hot, so it wasn't unusual to place a heatsink atop parts that were particularly sensitive. This presented an opportunity to put easily recognized components or components that made it too simple to figure out what was being done and how it was being done, under a soldered-in-place heatsink. Sometimes, it's desirable to put what looks like RF shielding in place, too. If you're able to label a module with some unrecognizable stock number, that works well, too.

Having spent over half my life reverse-engineering circuits for various reasons, I've encountered some of these tricks. Presently, I like to hide components underneath others. That's often easy to do with a mix of socketed and soldered-in-place SMT components. People might be tempted to "buzz-out" your circuit, but they generally won't disassemble it.

RE