A good question.
I have 3 and I obviously like them.
For me it's 80% about the tone, that I love. Ric tone is not just a 'one trick pony' either but it's certainly not an ultra versatile do everything well bass. I love the nasal, clanky treble especially the way it sits in with the neck pickup as opposed to by itself.
The build quality is good, sustain also. There's something about the whole '50's American car design' about them and the fact that they've changed so little but still do their job so well.
They take a little effort in terms of they're not maybe the easiest basses to play but I find myself picking mine up as my favourite 4 string everytime. (ACG being my preferred 5)
My 4003 has the Vintage Tone Control and I wouldn't be interested in a modern 4003 without it, although it can be retrofitted.
My two 4001's are C series basses and so have that vintage sound too.
Why are they so expensive? They are made in the US with US electronics in a small family firm. As to the price of the older models, you could ask the same question in relation to say Fender or Gibson basses.
Idiosyncratic yes and I understand why not everyone likes them but mine keep shouting 'pick me up, play me' which is great.
Peter