You can get caught actually, and let me just say, it is not worth the repercussions. If you’re going to buy a new iPhone (or any product over the threshold) while overseas and not declare it when you return to your home country, you accept you’re doing the wrong thing and taking a real risk. Not get caught, sure, you get away with it and it most likely won’t ever come back to bite you, but if you do get caught, you’re in some pretty hot water.
I actually want to chip in with this, my (European) cousin bought a 6s back in its day while on holiday in the USA. Upon returning to Europe, she decided to take a risk and not declare it at customs - an officer asked her for a random check afterwards, as soon as they questioned her “...where’s the phone from?”, yeah, she got done big time. Not only pay the tax, but also there was a huge fine that came on top and time wasted at the airport. Plus some really hostile interrogations from the customs police - just trust me, it’s not worth it