I would imagine that in the early days there was nothing that met the requirements. Early on very few systems had the drive density needed - products like Sun's Thumper with 48 drives (https://en.wikipedia.org/wiki/Sun_Fire_X4500) were few and far between.

Today there are lots of high storage density devices, so no point to build your own if you can get it already engineered and with a warranty from somewhere else.