Most-searched for keywords will also typically be the most competitive.
Competition = higher CPC and lower CTR. So, you're spending a LOT of money on keywords that have a low chance of getting a click (when they do, it's an expensive one at that), and is likely to be early in the buying cycle (less chance of a conversion).
For example, "real estate" would not be a desirable keyword. If you're selling luxury apartments, what percentage of people who search for this are looking for what you're selling? If you were to bid on this, you're going to pay big for it and see no real conversions from it.
However, "luxury condos in highland park" with a geo-focus on that area will not get a ton of clicks, but the conversion rate will be extremely good on a well-written and optimized landing page. Not to mention, the clicks will be cheap; probably 10% or less than "real estate". Although that keyword is one that wouldn't even show in the AdWords Keyword Tool, but is highly relevant to someone looking for what you're selling.
It's these sort of things that can't be taught easily, but that come from experience and independent research.