There are a number of reasons why ants and insects can climb walls, but not humans or mammals. Here are they;

i. They have very tiny spikes/hooks/thorny projections/hair on their limbs, which often is attached with hard claws. They help in increasing friction between insect's body and wall (Which looks plain or frictionless to our naked eyes, but is actually not. It has plenty of foot-holds; big, nice and easy enough for a tiny insect) and also fit well in the tiny pores or holes or projections (foot-holds) in the wall. This helps them climb.

ii. They have hard and powerful exoskeleton. This feature is a remarkable powerful thing. It gives them enormous strength (as compared to their sizes) to leap, jump, run and climb. Researches are going on to incorporate this feature in soldier's outfits so that they too can avail such powers.

iii. Those who have wings get a lift by using them.

iv. Insects like spiders use their web to climb in addition to other features discussed above.

v. Insects have very less mass. This results in very less effect of gravitational pull on them, making their weight very less.
Weight (Force) = Mass X Gravitational acceleration (9.8 m/second square).

vi. Finally, wind or air plays a vital role in lifting them. Since they are very light weighted, air or wind acts a sort of buoyant force for them.

vii. Some insects have sticky limb-ends or body, which help them climb.

viii. Some insects also use vacuum pads (very rare cases).

We or big animals cannot do that because we do not have these features.

So, that was that. Hope it was useful for you. Thanks.
Source(s):
Studies on zoology and antemology

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