Anything involving Apple is a bit nuts.
But among other things, an Apple I represents one of the early beginnings in the computing world. You can't get much closer to the direct early work of Woz and Jobs. Apple Is are indeed very rare. Supposedly most Apple Is were traded in for discounts on Apple IIs, so few were left in the general public. The parts themselves are rare. Woz chose some parts that weren't even incredibly common at the time - so it is difficult to build accurate replicas. (An Apple II replica, in contrast can be built from mostly from parts that are still available new)
There is actually quite a bit of demand for vintage items that similarly represent various "beginnings". Such as TRS-80s, Commodore PETs, Apple II/II+s, IBM 5150 PCs, etc, but those are common enough they can often be had for a few hundred dollars.