Some customers think they can build everything themselves and consider it as an advantage over their peers.
The reason for it is very simple: Sunk cost and the Ikea Effect.
The sunk cost of their knowledge and skill. How can it go to waste? Even though this is no more our primary task.
The Ikea Effect because we have built it, so we are incentivized to use it even though the product is a hack.
A sales guy should not waste time on such customers. People will hire a donkey for the job of horses because it is their donkey. You cannot change their perception.