For a long time, large banks defaulted to building. They had the engineers, the budget, and the conviction that no vendor understood them well enough. The trade-off was acceptable.
What changed
The surface area exploded. AI, fraud, payments, regulation, data — every domain now demands a specialist team. No bank can staff every front.
The pragmatic stack
Build what is genuinely differentiating. Buy what is critical but not unique. Adopt an execution layer that lets both coexist.