I know this may seem like a stupid question to some, but it is something i have been wondering about for a while....Why is it whenever you send a fleet out, it curves on its flight path in a arc, instead of just flying straight from its point of origin to its destination as you would expect?

Well.... no question is stupid!
Answers can be!

None from the game designers/developers/admins/partners bothered to answer this. Either they do not care, or they do not know what the designer-Overmind had in his mind.

The line is curved because universe is curved. Provided that universe is still expanding following the Big Bang, the Overminds of the game wanted to give a realistic touch to it.

Of course, realistic is not necessarily logical, but this is a case to be discussed separately.

p.s.
Those are purely personal opinions deriving from logical (!!!) thinking. None of the opinions are explained to me from game admins, and I am not in anyway involved with game decisions.

Dont know if this helps but most space flight is curved and uses gravity to sling the ship around.  The apoapsis is the high point in the orbit and the periapsis is the low point.  The game "Kerbal Space Program" is awesome about learning the way space flight really works (as far as we know about it today).