The reason why objects without a heat shield or entering the atmosphere uncontrolled incinerate is simply due to the fact air can't get out of their way fast enough. The Space Shuttle strikes the atmosphere at 25 times the speed of sound, and objects coming in from deep space can be moving three or four times faster. As an object encounters the atmosphere, air cannot move out of the way due to the hypersonic velocity, and instead it piles up ahead of the object and compresses into a shock wave. The gases there are compressed rapidly enough to heat up to 20,000 degrees or more, and because of that the radiant heat from the shock wave starts to heat up the incoming object. The heat will either melt or vaporize any material exposed to the shock wave, and since aluminum is used heavily in the construction of spacecraft, the spacecraft tears apart and the pieces melt or vaporize. The shock wave also creates drag upon the object, which slows it down as well and created immense pressures on it as well. If the object has a heat shield, it either melts and burns away, taking the heat with it, or like the Space Shuttle's simply insulates the skin from the heat. This is possible because instead of just diving in like a nuclear bomb or skipping along the atmosphere like a rock off a pond as the Apollo lunar missions did, the Shuttle rides the upper atmosphere like a surfer. That keeps the temperatures low enough for thermal tiles to protect the spacecraft during re-entry. Other spacecraft make a ballistic re-entry, which not only creates hull temperatures of 6,000 degrees and higher, it also treats the crew to severe and sometimes near bone crushing deceleration too. The spacecraft require a very heavy heat shield that cannot be reused, and it burns away to save the spacecraft. The re-entry must be made at a precise angle, or the spacecraft will burn up like a meteorite, or even skip off the atmosphere to careen off into space again. A spacecraft must make a controlled entry no only to avoid breaking apart and burning up, it's also to keep the gee forces from killing the crew. As for a man in a space suit, he couldn't hope to survive a re-entry in one. Even if he didn't burn up, he would hit the ground at several hundred miles an hour without a parachute to bring him to a safe landing. The idea of making a re-entry in this way has been considered, but shelved in the 1960's.