Question

why is the weight of an object at a high altitude less than its weight at the sea-level

Answer 1

this is because, when the altitude increases, the distance between the object and the core increases i,e if the distance between these two were to be taken as the radius (R), the the (g) decreases as the (g) is inversely proportional to (R), thus reducing it's weight as weight is directly proportional to (g)

Answer 2

because according to Newton's universal law of gravitation force is indirectly proportional to distance squared and as distance increases gravitational force decreases.
so let the gravity at sea level be G1 and gravity at high altitude at G2
Weight = mass x gravitation acceleration
Weight1= mass x G1
Weight2= mass x G2
as G1>G2(distance is less gravitation is more)
therefore W1>W2.