I assume you mean the kits with the brown plastic trees and foliage made of a spongy material in red, green, yellow, etc. I just shape the tree, put on a heavy coat of white glue and drop the tree in a bag containing the foliage and shake.
I agree this does not make for realistic looking trees. To do that will require the addition of paints for blending and detail. Like the tree trunk itself. Real trees are not brown.
The Woodland Scenics trees are like our models in the sense that you can't just throw them together and expect to make something that looks realistic. It requires lots of extra work.
Some of the most realistic looking trees I've seen are the handmade ones using commonly available materials.
Also keep in mind these need to be viewed from a distance. Looking at them close up will reveal all the flaws.
You Tube is your friend!