1 - they operate from stationary, extremely obvious and vulnerable bases,
2 - these bases, and the aircraft operated from them, are extremely reliant on trained manpower, which is extremely vulnerable to attack,
3 - their response time is the highest of all combat arms,
4 - their granularity is the lowest, being composed of a small number of extremely expensive platforms,
5 - they are the easiest form of weapon to detect and attack,
6 - they are the most vulnerable form of weapon to electronic jamming and attack,
7 - they are reliant on intelligence gathered by other sources and themselves can only usefully gather Elint, and even then only on modern and expensive platforms,
8 - they can not hold ground,
9 - in addition to their bases being stationary, combat stores cannot easily be moved from base to base. A flight of F-15s will use as much fuel for one combat sortie as a company of tanks for a whole days consistent fighting, or 500 kilometres of march. A 500kg bomb weighs the same as 10 155-mm shells, and so on.
that is only the list of military, and not economic or strategic problems with aircraft. Over the years, air forces around the world, including the USAF and the RAF, have lobbied intensively for undeserved funding claiming some mystical ability to decide the fate of entire conflicts. They have guaranteed results that have fallen completely short, have lied to government authorities about their capabilities, destroying entire national aircraft carrier programmes in the process. They have performed expensive and pointless missions to prove their existence. And when faced with actual opponents who did not fold over and die at the first moment, completely and utterly failed to accomplish either their objectives or achieve a decisive victory.
But of course you can bomb Iraqi tanks on a retreating road march. Of course they will die. Or you can attack Egyptian army operating in desert out of SAM and fighter range. Of course they will die. But if your opponent decides to fight back...
What happens then?
Allied air forces were unable to decisively effect the outcome in Korea - Communists fought back in the air and prevented allied aerial victory. US bombing failed to stop North Vietnam - Vietnamese just kept doing whatever they were doing. US bombing failed to dismantle the Serbian ground forces in 1999 - Serbian ground forces emerged essentially unscathed. US and Coalition air forces failed in dismantling the Iraqi ground defences in 2003. Good job the Navy was there with their cruise missiles or the air forces may have fucked up even more. British RAF did huge cross-global bombing mission on Falklands and achieved precisely nothing. In previous decade they tried and succeeded in destroying the Navy's carrier plans but the Navy kept some carriers. If they hadn't, Argentines would have had total instead of partial air supremacy. Game over. Israeli aircraft tried to stop Syrian tank advance and were cut to shreds because Syrians brought anti-aircraft weapons this time.
The main utility of air forces is to attack strategic positions which are out of the range of artillery. Wars are won either at sea by naval forces or on the ground by ground forces. Examination of all conflicts where neither side was able to attain obvious air superiority will show this to be true (four quick examples: North Africa, Korea, Iran-Iraq, and Yom Kippur opening stages), and it becomes more true as technology advances. Modern militaries are phasing out CAS for a reason. You will even notice that the attack component of JSTARS was not aircraft, but ground forces.

He can hide. He can run. He requires a minimal amount of mobile fuel stores to be moved and can reload and fire again within a space of minutes. He can scatter bomblets or chemical weapons over an entire airfield or attack and destroy with precision fuel or takeoff/landing/communications facilities from 300km.

This guy can do the same, from a longer range (with a correspondingly longer reload time, though).
edit: this all applies to land warfare only.





