because they wanted to
Arches are a very strong support for a building foundation. It is just natural that they use it in their architecture as it was the strongest they knew how to build.
Yes, Constantine did build the Hippodrome of Constantinople.
what you do is take aboutn twwo packs of Q-Tips adn glue them together end on end .As you go slowly curve it making the arch design or shape.
The Romans built the arch of Titus as a memorial to the long, hard won victory over the Jews.