Why can't the party go around Mirkwood?

As Gandalf said, it would take weeks and weeks of trekking out of their way to go around Mirkwood. Hundreds of miles out of their way. It would be much too bothersome and time-consuming, rather than just cutting straight through it.
It was too long of a trip and it would have taken them past Dol Guldur to the south and orcs or dragons to the north.