Legacy software is successful software. What's the point of building a program that will be discarded after a few months because its users fled to the next big thing? That just means more lines of code being written overall, needlessly.
Legacy software is software that served a purpose but no longer meets the business objectives. Some common factors that influence the decision to replace a legacy system with a new one include: maintenance cost, scalability, flexibility. The usual approach in startups is to build something at minimal cost, then replace it later if the business requires it. Everyone has a different opinion regarding the importance of these factors at various stages of software life.