Probably because that rucksack has his underwear, toiletries, shirts, pants, socks, an extra pair of shoes, a rain tarp, compass, flint and steel, canteen, first aid kit, a coat or two, a bundle of sage in an air/water tight hard side container, survival knife, hatchet, waterproof lighter, and ~11 lbs (5 kg) of ammunition in various calibers.

In short, Clive packs with the intent of walking into the woods and not walking out for a good while.