Roman block lettering avoids curves because it was often curved. That's why the U and V sounds were represented by the same letter, and only later diverged. Norse runes are similar, and lack curves entirely.
It doesn't really avoid curves, though, even when carved into stone. Look at the inscription on Trajan's Column [1], f'rinstance; yes, it's got the classic U written as V, but the C, D, O, S etc are all as curvy as you could possibly want.
The Romans wrote most ephemeral stuff on wax tablets which could be smoothed over for re-use; this is where the phrase "tabula rasa" comes from. Some examples survive and also show curves [2].
