According to the FAA[0], the top ten leading causes of fatal general-aviation accidents between 2001 and 2016 were

1. Loss of Control Inflight

2. Controlled Flight Into Terrain

3. System Component Failure – Powerplant

4. Fuel Related

5. Unknown or Undetermined

6. System Component Failure – Non-Powerplant

7. Unintended Flight In IMC

8. Midair Collisions Low

9. Low-Altitude Operations

10. Other

Items 5 and 10 give us no information. We can charitably let pilots off the hook on 3 and 6, but the root cause might be inadequate preflight inspection or maintenance.

The rest are clearly pilot error.

(1) Loss of control is commonly (aerodynamic) stalls that sometimes lead to spins, and a large number of those tend to occur on the same turn in the airport traffic pattern just before landing.

(2) Controlled Flight Into Terrain (CFIT) means the pilot flew the airplane right into the ground, a mountain, buildings, or water. The pilot failed to recognize or prevent an impending collision.

(4) Fuel mismanagement is one of the most preventable on the list. Take enough fuel for your mission plus the required legal reserve! Math is hard, though. If I have fifty-gallon tanks and burn 16 gallons per hour, that’s four hours of fuel. In other cases, impatience or being a cheapskate not wanting to take on fuel for a short mission is the culprit. Arrogantly proclaiming “I know this airplane” will do it too. Pilots say the only time you can have too much fuel is when you’re on fire.

(7) Unintended flight into Instrument Meteorological Conditions (IMC) means accidentally flying into clouds, fog, precipitation, haze, and so on. Flying in IMC requires an adequately equipped airplane (A TOMATO FLAMES plus GRABCARD[1]), an Instrument Flight Rules (IFR) clearance from Air Traffic Control (ATC), and an instrument-rated pilot with IFR currency. This means additional training, preparation, and proficiency. Without outside reference to the horizon, humans are subject to all sorts of dangerous sensory illusions. An instrument-rated pilot must navigate solely by reference to the flight instruments, not an easy skill. A study with dubious methods concluded that after unintended flight into IMC, a pilot has 178 seconds[2] to live.

(8) In Visual Meteorological Conditions (VMC, the opposite of IMC), all pilots have see-and-avoid responsibility for obstacles and other aircraft. Pilots on IFR flight plans are guaranteed ATC services. Pilots on Visual Flight Rules (VFR) flight plans may optionally obtain VFR traffic advisories, also known as flight following. When aircraft are close together, ATC will point out traffic (other aircraft), e.g., “Cessna 123AB, traffic two o’clock, five miles, six thousand, southeast bound.” A collision in IMC might be due to ATC error, namely loss of required aircraft separation, but those cases are the exception.

(9) Low-altitude operations means flying near the ground. A common example of this is crop dusters. Those guys are crazy. Hotdogging pilots buzzing their friends on the ground is a foolish example that leads to accidents. Think of YouTube videos of someone in a Cub flying low over a river with trees on either side. As you noted, this is an extremely low-margin environment. Over a congested area, pilots are required to remain at least a thousand feet above the ground, which is still extremely low. Sparsely populated areas and open water have no legal restrictions, and the rest require at least a 500-foot buffer. All of those are still extremely close to the ground and require a sharp pilot. Power lines, antennae, towers, bridges, and trees are all potentially fatal obstacles.

[0]: https://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=2...

[1]: http://www.askacfi.com/1651/ifr-required-equipment.htm

[2]: https://www.youtube.com/watch?v=fXzYZjpoz_E