You must not own any fine denim jeans.

Take for example a pair of old, Rock & Republic jeans (they don't make them like this anymore since Kohls bought the brand). So these jeans are made of very thick, yet soft denim. It's a mix of fabrics that make it durable yet very comfortable. If you reverse the jeans you can see the threading and how fine it is. The denim is dark, yet the reverse almost shimmers. Then you see the stitching. Usually these jeans are sewn throughout with a very thick serger stich. The thread is fine, so on just the serger, up to three times more thread was used than on your average jean. Not to mention this thread is of very good quality.

Then you see the stitching on the outside and notice that it varies in color. So they used two different colors, one inside and one outside, it takes time to do all of that. On the outside stitching there is another type of very thick stitch. For this they use a more expensive type of thread.

Then you notice the rivets and other metal details. The zipper is usually of a slightly different color, not your average navy blue, run of the mill zipper. The zipper fabric is also softer than most zippers. The rivets are of hard metal with the logo engraved, attention to detail is awesome, as you can clearly see the skulls even though they're tiny.

Then you see the tag. The tag is made with very soft, fine fabric and with lots of thread, if you turn the tag around you'll see just how much thread goes into every tag.

The fit is also perfect. The cut of the jean is very flattering and has a unique style, ever so slightly curving at the right angles. After you wash it many, many times, year after year, the color will not fade and the jean will not lose shape.

So, a couple of things make designer jeans "better" than your average Levi's: cut, specialty denim, more stitching/threading (often in various colors,) expensive thread, specialty metal details, and of course durability.

Having said that, if the jean is no longer made in the US, I question its quality. After a brand gets really big, they then start manufacturing in Asia to save money and then you end up just paying for the brand name. Who knows, haven't bought Trues in over 4 years.

TL;DR: I'm a fashion designer passionate about jeans, here's why.