Why do most meteoroids burn up in the atmosphere?
Most meteoroids are tiny, and they hit the air so fast that it is compressed and heated in front of them, melting and vaporising their surfaces. Small grains are destroyed completely within seconds.
Earth's atmosphere is a remarkably effective shield. Most meteoroids, the space rocks and grains that hit it, are no bigger than a grain of sand, and they arrive at between about 11 and 72 km/s. At such speeds the meteoroid cannot push the air aside gently. Instead, a layer of air in front of it is rammed together and heated to thousands of degrees. Contrary to popular belief, it is mainly this compression, not friction, that does the damage.
The intense heat melts and vaporises the meteoroid's surface, which is stripped away in a process called ablation. Hot vaporised atoms and air molecules glow, producing the streak of light we see as a meteor, typically at heights of about 70 to 120 km. A sand-sized grain has very little material to lose, so it is gone in a second or two. Even for larger rocks, a large share of the original mass is usually ablated or blown off as dust.
A second destroyer is pressure. The force of the oncoming air grows as the meteoroid plunges into thicker layers. Many space rocks are cracked, crumbly or loosely held together, and they shatter when that pressure exceeds their strength. The fragments then ablate even faster.
To survive and land as a meteorite, a body needs some combination of being fairly large, strong and arriving relatively slowly. Iron meteoroids are tough and survive well; fragile comet dust is almost always destroyed, which is why showers like the Perseids drop nothing. The survivors slow to free-fall speed high up and fall the rest of the way unlit. So of all the material that enters the air each year, only a tiny fraction ever becomes a rock someone can pick up.