[apache]

Thats a much simplifed, but more or less accurate description of orbital mechanics Barnacle Bill. Anyways, by doing orbital transfers, you save an immense amount of fuel, and thus spacecraft mass. That is the big deal with 20th century (and 21st century) space flight. You actually could fly a direct route to Mars from Earth, but it would mean expending a lot more fuel, though it would mean you get there much quicker.
However, 25th century space flight (according to the game) apparently uses some form of non-intertial propulsion, meaning fuel and spacecraft mass is irrelvant, and you can turn on a dime.