one reason not to arm Vnav on the ground, could be it the airport has special convoluted noise abatement procedures, ie aircraft is light, and you dont want to chase 230kts but instead stay at up speed (and you dont want to use SPD int)...
or more likely because you want to delay part of the flap retraction and keep lower groundspeed, to cater for adverse strong tail/X wind turing a tight turn on departure... (thinking about those places with rocks hidden i the clouds)