there are multiple short-acting and long-acting insulins because you can't patent other people's things, but now it's all off-patent. just take your stainless steel bioreactor and preparative HPLC, cook up a batch, wait ten years for biosimilar approval and you're good to go

because unlike with small molecule drugs, when cooking up generic biopharmaceutical there’s extra approval process that amounts to a tiny clinical trial https://en.wikipedia.org/wiki/Biosimilar this and type of economics of scale that there is with biologicals makes manufacture at large scale way more preferable. these requirements were loosened a bit over time