Why 18-135mm or 18-140mm lenses are so costly compared to 18-55 and 55-250 kit lens

I am thinking to buy new dslr camera with some good lens. I saw various lens and various brand and one thing I don't understand that why these lenses are so costly compared to other lenses. I know there are many other costly lenses but if I make comparison with kit lenses which comes with brands like Nikon, Canon, Sony, Pentax.. all with 18-135 or 18-140 have kit lenses so costly that they actually cost more than combined price of 18-55mm + 55-250 kit lenses. It is with all brands, not just canon or just sony or nikon. And I am very much confused to make decision because the camera I want to buy gets super costly with 18-135mm lens variant. And if I decide to go with combo of 18-55 and 55-220mm kit lens then its cheaper but folks in camera field advice against it.

So I want to know why they are so costly, because kit lens are of generally same quality but change in focal length causes so much change in price . At times it feels like they are making us fool with such a pricey lenses while others (combo) is cheaper. What makes them so costly.. focal length or they have better build ?

I think it is because to maintain quality at higher zoom as well as lowest zoom, they need more engineering and precision. My wild guess is that 18-55 mm has not much variation so its easy to maintain quality so they are cheap. Whilst, 18-135mm means longer zoom lenght which starts from just 18mm. So, this is I think something to do with the way lenses are made.