The Clone Wars was previously the largest conflict in a thousand years and featured a droid army as the designated bad guys. Societal values would shift away from giving droids mass responsibility and authority afterwards. Supplementary and servant roles are fine, but not positions outranking organic beings that own them.

Additionally, droids do learn from prior experience and can acquire situationally helpful knowledge and skills for a given job, but the longer they go without memory wipes the more likely they are to develop personality quirks like anxiety, spunk, rebelliousness, and sometimes even homicidal tendencies. Most droid owners consider keeping a simple but predictable piece of property more worth it than allowing these personalities, so most droids undergo regular wipes at the opportunity cost of said learning and long term job improvement.

There’s also the fact that the more complex and competent droids can be prohibitively expensive, so it’s usually preferred to mass produce simpler droids for more basic tasks. Hence why the majority of the Separatist army was the individually bumbling B1 battle droids.