I will try to summarize why the monk is mechanically weaker than others.
1. Their starter abilities feel like 'image abilities'. Unarmored Defense is a substitute for wearing armor, but its not better than actual armour. The whole point of the ability is to look like you're not wearing armour, but have the near equivalent AC of armour. It hinges of 2 stats, which means a non-min maxer will find their AC to be quite low, and have to spend lots of ASI improving it. At high levels its a very weak feature when you have to pass up powerful magic armours. Martial Arts is to look like you dont have weapons, but deal the equivalent damage of weapons. It starts off about the same a dagger's damage and slowly increases, but its not better than using actual weapons. Actually its a fair bit worse than weapons, since it doesn't have any feat support. It also suffers the same problem as Unarmored Defense, which is you have to pass up powerful magic weapons, or are limited to Monk weapons. Ki-Empowered Strikes is given because they realise this problem.
Unarmored Defense and Unarmored Movement are often compared to the Barbarian's version, which is less restrictive for some reason. Barbs can choose to wear medium armor and use shields to have better AC and still have the bonus speed and still can Rage. Monks are however stuck with a certain image.
2. They have d8 Hit die. So from the above we know Monks deal low-ish damage, and have mediocre AC. D8 is not great, but most melee classes are D10, except Cleric, but they are full casters and usually have heavy armour. Rogues occasionally get in melee, but they have Uncanny Dodge and bonus action hide to help survive. When Monks get hit, it hurts.
3. They benefit little from Crits, but suffer greatly from misses. Low Martial Arts damage die means less extra damage from crits, but each miss is one less Stunning Strike chance.
4. Their main selling point, high movement speed, is not that important. Speed is nice for closing gaps. Once the gap is closed, its a slugfest, and Monks aren't good at slugfests. Speed is good for running away, but there needs to be lots of open space for that. And Mobile is mandatory.
5. Their high level features are weak. Are you looking forward to walking on water, Tongue of Sun & Moon, Timeless Body, and Perfect Self? I am not. The monsters you face get stronger, but at least you can't be aged magically, and can save on those ointments for your backpain.
6. Stunning Strike alone deserves discussion. At low levels, it can stun-lock monsters until its dead, making single strong enemies a non-challenge. At high levels, most monsters have massive bonus to CON saves on top of Legendary Resistances. Trying to burn legendary resistances could back fire by burning through your own Ki.
Overall, Monks are just not good enough when compared to other martials. Their damage is low, and their utility is non-existent outside of Stunning Strike. They are also a bit frail. They are also restricted in equipment and feat choices. They are very fast though, but that means they are better off being a courier, messenger or Grab delivery man.
EDIT: I think Monks are unsatisfying to play also because they get less support from their allies. That Sharpshooter Fighter or GWM Paladin is obviously more deserving of Haste and Bless than the Monk.