I've been asking that question for several years now. The fact is that the people who created them like SH3 and see no need to change. The people who have the skills to convert them for SH4 also don't seem interested. A GWX team member was working on some cool harbor updates when GWX4 got cancelled.
As for SH5, it's so different that I'm not sure anything there could be truly imported into SH3.
So we wait, and we hope, and we dream.