Naming the phenomenon
Scope creep is the incremental growth of a project's deliverables, requirements or ambition beyond what was originally agreed, without the budget, schedule and approval being formally revisited. It differs from a managed change: the change is visible and decided; creep accretes below the threshold of decision.
Why small yeses compound into a different project
Each addition passes the local test: it is genuinely useful, it is small relative to the whole, and refusing it feels obstructive. What no individual yes triggers is the global test, whether the project as now constituted would still have been approved at its current cost, duration and risk. Creep is also rarely innocent on both sides. Suppliers on time-and-materials welcome it; internal teams use a funded programme as the vehicle for every wish that could not win its own budget. The result reaches steering as a delay or an overrun, when the true event was that the decision itself changed and nobody re-made it.