It would appear that creating a form sufficiently stable for Palpatine to inhabit was well beyond the capacity of the Sith Eternal. This lack of ability presumably includes building an army of dark side Force adepts.
The heretics of the Sith Eternal toiled, splicing genes, bolstering tissue, creating unnatural abominations in the hope that one of these strandcasts would succeed and become a worthy receptacle. The heretics would do anything, risk anything, sacrifice anything, to create a cradle for their god-consciousness.
Nothing worked. But their efforts were not entirely in vain.
One genetic strandcast lived. Thrived, even. A not-quite-identical clone. His “son.” But he was a useless, powerless failure. Palpatine could not even bear to look upon such disappointing ordinariness.
Star Wars: The Rise of Skywalker: Expanded Edition
One of the central conceits of the Star Wars universe is that while cloning is possible, doing so without having the original donor around is extremely challenging. We also see (mostly in Legends stories and video games) that cloning someone with Force powers is doubly difficult.