Never seen a 7-series that wasn't driven by the owner.

My local council had one chauffeur-driven for the Mayor and associates until the need for a more 'austere' public image saw it being replaced by a Skoda Octavia ( Jetta / Golf platform ).

I think the Mayor of Belfast still rides in the back of a 7.

Not the base one, that's true. The "Lang" versions are made for sitting on the back seat.

The "I don't drive myself"-version of e.g. the Audi A8 is the A8L. Same for the BMW 7. (At least they used to have e.g. the 740L).

But both the S-class and the 7-series are very VERY rare luxury cars. I see thousands of 5-series and E-classes for every 7-series or S-class. The difference in price must be huge in europe compared to the US, because there I saw tons of 7-series and S-class was almost as common as E-class.

Yes, the price difference is huge. E class is common as a taxi here, base price for a consumer model is 55K Euros, S class nearly double that.

What’s the reason (apart from tradition making people think it’s normal) the proce difference between these models is so big in the EU and not in the US?

Taxes. I drive a 15 year old car, I would not want to waste money on a new one. My car cost about 14% of what it was new with 55K km on the clock and is in a much cheaper tax bracket than a new one. It's a ridiculous system.

This may be true in China, but not most other countries.