Because you cannot test it or observe it.
A scientific theory are created through the scientific method, in the scientific method you need to be able observe it and experiment.
Very powerful microscopes were needed before cells could be observed in detail
<span><span>Clouds of gas form within galaxies.
</span><span>Formation of structure within the gas clouds, due to "turbulence" and activity of new stars.
</span><span>Random turbulent processes lead to regions dense enough to collapse under their own weight, in spite of a hostile environment.
</span><span>As blob collapses, a disk forms, with growing "protostar" at the center.
</span><span>At the same time, bipolar outflows from forming star/disk system begin.
</span><span>Material is processed, moving in from the blob to the disk. What is not lost in the outflow builds up on the protostar.
</span><span>When the protostar begins to undergo fusion, it becomes a real star.
</span><span>Once the outflow ceases and the "accretion" phase that lead to the buildup of the star ends, a disk of "leftover" material is left around the star.
</span><span>At or near the end of the star-formation process, the remaining material in the "circumstellar disk" (a.k.a. "protoplanetary disk") forms a variety of planets.
</span><span>Eventually, all that is left behind is a new star, perhaps some planets, and a disk of left-over ground-up solids, visible as a "Debris Disk"around stars other than the Sun, and known as the "Zodaical Dust Disk" around the Sun.</span></span>
