Because integrations bind to the API name, not the label. Renaming a label is safe and cosmetic; changing an API name breaks every workflow, report filter and external integration pointing at it. Treat API names as a contract you don't renegotiate after go-live.
Before changing anything structural, find every consumer — reports, list views, flows, sync mappings, external tools. Most CRMs give you a dependency view; use it, and keep a field inventory with an owner. This is also why the migration field map is treated as a gated deliverable rather than a working note.
Every claim above comes from work LeanScale published. These are the sources.