Romans *did* write with macrons! Video essay on Latin Apices & Hidden Quantity
This is an interesting video https://youtu.be/D3bmLi1bKI0
The Romand did use apices sometimes. It's true. The mark served the same function as macrons do nowadays.
The Romans did not use them always. This is also true. But, and this is what these discussions usually miss, that is because the Romans already knew how to pronounce the language correctly. Macrons are useful today to guide people to pronounce Latin correctly, because people nowadays do not know how to pronounce Latin at all before they start learning it.