That's already the case. This is why most drug companies are quite large. And investors spread their investments. Competing as a small startup is extremely hard

That's already the case. This is why most drug companies are quite large. And investors spread their investments. Competing as a small startup is extremely hard compared in this field compared to for example software. Unless you're asking why there is not a single state-run drug company doing everything, in which the case the answer is scale has disadvantages, having some diverse set of companies in a space like this has advantages, and state run centralism has been debunked so many times I can't be arsed.