He buys gear if it is a better item than what his sponsors provide.
Or he modifies it.
And he does start with a much heavier pack when including the food and
water. Most folks doing this sort of utlralight trip for long
mileage in new territory have around 45 lbs with food.
They also talk about what stuff weighs in the pack, in their pockets and the total "skin out weight".

11 more oz can be another half day of food ie. 15 more miles
without resupply. In 6000 miles this could save you 20 days
of resupplies, not counting extra miles hiked out to the resupply.

That's 20 days less exposed to the elements, away from family,
faster than the competition etc. Not saying everyone should do it,
but why people consider leaving behind some elements of preparedness for greater speed on the trip.