Monk class mechanics go against core combat design
The reason that the monk tends to fare poorly in practice is that the class cannot in any real way benefit from the same things that other melee capable classes benefit from.
To put it simply, high level non spell casters are very equipment dependant to remain competitive, and the Monk is designed that not only is it intended to not use equipment, but it gets penalized if it tries to.
A 20th level Fighter, Paladin, or Barbarian will have magic armour, maybe a shield, and definitely a weapon. He also has a high Bab, so he can plow through the AC of his opponents and deliver damage.
A 20th level Rogue wont have the BAB, but he can still use much of the same equipment, and it can make up for the low Bab by flanking and inflicting scary levels of sneak attack damage.
A 20th level monk will have a large number of attacks, great saves, and lots of neat abilities. However, he will have a low Bab, and if he uses magic weapons, he loses his high damage output. He cannot wear Armour if he wants to benefit from his wisdom bonus to AC. And unless he has a great many stat boosing items, his AC is going to be much lower than a comparible warrior type. He does not even have the BaB to use the Expertise feat to mitigate the low AC.
The Monks primary method of killing opponents is to use melee attacks, and there is no point at which a Monk can function effectively against level appropriate opponents.
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