My experience with academics is that most don't know / don't understand the benefits of coding things in a better way, so they don't even consider hiring someone to help.

The other issue is that the incentives aren't necessarily aligned. They don't get payed to share their code or to make their code reusable. The code is used to publish one or a couple of paper and then thrown away. Maybe someone else will reuse it in the future but the original author doesn't necessarily gain much from that