Microgreens are just early versions of regular herbs and veg, cut before they’ve had a chance to grow. As the goal is micro-sized, trays are sown with loads more seeds than would be sown if growing the full-sized thing. The results are punchy, nutritious greens that can grow indoors all year-round.
Growing greens for a few weeks is pretty fault-free, there’s just not that much that can go wrong in that space of time. Veg that you wouldn’t grow indoors or you’d wouldn’t want to grow bigger can be grown to a small leaf all year-round indoors.