Lots of classes in older D&D were not balanced to each other, and not balanced at every level of play.
One of the main advantages that 2E monks had with the wacky unarmed combat tables was decent odds of getting knockouts or stuns - they had better chance of bumping up and down the results on the table, giving you some advantage there or of pinning/restraining some enemies with wrestling. (of course, that requires a creature that can be knocked out or stunned, or a creature you can actually touch as opposed to a ghost or a pool of green slime...).
Being able to either stop the enemy from acting at all, or skipping the hitpoint buffer with a knockout became more valuable at higher levels, but of course, you're basically a glass cannon gambling on random conditions. Normally, you'd want to be buffed in every possible way from your mages and magic items you've found or created to improve your AC and so on.
That said, mostly the monk was depending on you having outrageous stats and being very lucky.
The thing to realize about stat requirements is that unlike later editions, the default for play was random roll, and many groups did not let you swap around scores - so it was complete luck of the draw as to which stats you got high vs. low. So, instead of looking at it and saying "Oh, the monk would have a sucky Strength & Dexterity" it was really more "I need to have been lucky enough to roll GREAT Strength & Dexterity AND made these required scores ON TOP of that."
Now, the reason this even works this way is that way back, players were randomly rolling up an entire war party of several characters. The stat requirements basically meant most of your band would be Fighting Men, and everyone else who did something specialized or cool had more or higher requirements and therefore you had less of them. Monks, way back, were super powerful, and so the odds of rolling stats good enough to get one were very difficult.
Most of the time, you'd be short on the stats for a Monk, and you'd pick something else instead.
