Very few verses were actually abrogated
The reason is contextual, some events during the 23 years went through a change and hence needed new ruling. For eg, Allah commanded the Muslims to give sadaqa when they came to ask the Prophet private questions. Only Imam Ali did, and hence the verse was abrogated to allow them to ask without Sadaqa
The Quran has many levels of understanding and complexities as it’s the word of God, and His wisdom entails that a very few is abrogated. Also it highlights the need of the Prophet and Ahlulbayt to actually tell us which verses were abrogated, since amongst different schools disagreement exists in this regard