Out of custom and habit. Because it was there. The Air Force was only too happy to provide the transportation, so quite understandably the chicken availed himself of the opportunity. His wife lived across the road. He was solving the cross-road puzzle.
All roads are crossable by all chicken, but some roads are more crossable than others.
Because for every road you cross, there are ten more roads yet uncrossed. There are times for the chicken to cross roads and there are times to stay at the roadside. And then it crossed it. But still they pass its time. Because it could not resist the power of the Dark Side.