what does the iPhone offer that makes it compelling to not get a different device?

The hardware is just better. The software that Apple has designed works really well, and makes the hardware shine way better than any other device. I've used Droid 1 2009-2011, Galaxy Nexus 2011-2013. The hardware/software synergy is just much better on the iPhone.

It's a high end phone that isn't 26" diagonally so it fits in my hand and I can type with one thumb.

And Apple's after sales support is excellent. iPhone doesn't work properly? Factory reset. Still doesn't work? They'll swap the iPhone, in-store.