Because as articulated as a HT body is, a seamless body is much more. It is perfect for animating. When professionals do Stop-motion they usually use a metal armature. The seamless body uses a skeleton that is very much similar to the metal armature. Thus it is better to use for animation with far great range of motion. Which is my only/main purpose for the customs I make.
So if you have not tried animating your figure, you would not even notice the need for such articulation. I am the first to admit that from my experience, the best articulated body is the RAH Medicom Spiderman. I used it to make my animated movie. The only problem with that body is it is made of brittle plastic and is famous for breaking in the hip and shoulder area. I have not had those problem with the seamless bodies while maintaining the great articulation I need . So for those who want the very best articulation from there figures, I like to show there are other options. If that doesn't matter to everyone, that's fine too. I'm all about options. I hope you can see that now.