Google are not interested in conversions, they charge you per keyword click. They just want lots of people to come to your site and dont care if they buy or not, as they get paid per visit.
Lets say i own a restaurant and charge you more for a taster of every item on the menu before you can figure out what you like and can order it. If i charge you more for the taster meals, i dont care if you like anything, you paid a premium for the tasters, particularly if i have my regulars that buy up all the good food anyway. That is how Google operates.
When you set up a campaign you test many keywords and go through many attempts to see what works and what does not. That means many people come to the site but dont buy. Google make the same money if they buy or not.
It did not used to be like this but now Google charges you more at the start when you are testing keywords because they can get away with it and know you will keep trying and paying, before you give up on that keyword later realising it does not convert.