Why are Harbeth speakers expensive while its drivers are not.

Some of the best sounding speakers you could ever own. Lots of R&D and  time into the designs.  Price is based on performance and simple economics. They are worth it is because they sound better then competition at the same price point.  

+1 grannyring!

Grannyring is right. When the Monitor 40 got its first rave review years ago, they cost around 7k, now the latest version is over 14k. It wasn't that many years ago, and I should have bought a pair, but the point is that even then, they performed at a higher level than other 7k speakers, and if they didn't, people wouldn't be paying 14k now.

:@quanghuy
You should research other speaker manufacturers. It might surprise you how many also outsource their drivers. BTW,most won’t give that information. Hint: If they don't say they make their own drivers then you know they don't!

It's impressive when a company designs and builds their drivers in-house, e.g. Vivid, but in the end, it's the result that matters. Some designers make an art out of sourcing the best blend of drivers and the best crossover to couple them with.