Backlight bleeding is easy to fix, you just have to put opaque glue homogeneously on your lcd panel (on some devices you can do it yourself
by removing the panel and putting opaque adhesive tape on the rim).
But to achieve homogeneity every time is costly because it either needs to add an additional step in the assembly process (testing, be it manual or automated) and/or to slow down the process (to give the glue time to settle homogeneously and to dry properly). An additional cost (even if small in the grand scheme of things) that is only marginally beneficial because a very small minority knows how to detect backlight bleeding, an even slimmer one notice it, and an even thinner one cares.
An interesting anecdote : When the iPad 2 came out, you could know if a model had bleeding by it's serial numbers. If the serials were from a production run where the screen was supplied by Samsung you had little to no bleeding, but if it where supplied by Sharp it was a firework.