The critical consideration is that movement is dangerous, under conditions of pervasive battlefield surveillance, so when it is possible at all it must be exploited to its full potential, and thus can be conducted via an expensive process such as flight. In turn, to make movement dangerous to the enemy, one must make terrain frictionful to cross, through high lethality, low detectability assets, of which mines are the paradigmatic example.
[0] Brief bursts of flight interwoven between slow stalking of different ground locations for worms.