Why build it in-house instead of adopting an existing tool?

You walk the hiring manager through a project where your team wrote its own system. They challenge you: tools that do this already exist, so why not use one?

A good answer uses the real project: the options you looked at, the needs they could not meet (scale, fit, cost, control, time), how the decision was made and by whom, what owning the system has cost since, and whether you would decide the same way today. It shows you can defend a choice with evidence and also admit where it was wrong.