I see them as a bad way to earn money from only Advertising
Seems you answered your own question buddy. As far as I know you get paid for the captcha a small amount and the add sense as well as any space for advertisement. Unless you have very high traffic you won't get paid much.
Some faucet sites use AdSense, but most faucets use other ad networks specifically geared towards Bitcoin such as A-Ads, Adbit.co, or CoinURL.
And not all CAPTCHAs can be monetized. I don't think reCAPTCHA supports it (or at least not by default). Solve Media definitely does.
Most faucet sites get money solely through advertisements. These are usually either pay-per-click or pay-per-impression ads or affiliate ads. The meaning of first two types is pretty obvious - i.e. the faucet owner gets paid whenever a visitor clicks (pay-per-click) or views (pay-per-impression) an ad. Affiliate ads can only make money if a visitor clicks on the ad and purchases a product or service using the faucet owner's referral link. Then the faucet owner gets a share of the sale in the form of an affiliate payment.
Unless you happen to own a hugely popular faucet such as Freebitco.in, most faucets aren't very profitable which is reflected by the amounts which they dispense out.