My old boss used to have a long and very well-delivered spiel on the history of the wheel (literally) and its continual reinvention. One good way to make the point.

Plus by reinventing, you get to own your work, you learn the problem space, you develop processes for execution, you train your team, you start collecting valuable usage data, you position yourself in the market... and all those reasons are on top of the fact that you can probably make some kick-ass improvements.