There is reality to deal with
Without doing planetary flybys, then the probe would essentially have the kinetic energy gained by falling to the sun all those 10s of millions of miles, the fuel requirement to enter into orbit around mercury without taking advantage of other planets is HUGE...
This really is rocket science, and those working on it are the best at their job...
BTW there is not a 'straight shot' between planets, longer curved trajectories are always the most economical...