Uncarbonated liquids are dead simple to titrate, it's true. For a carbonated product like beer, it's actually a much more complicated problem than it seems. The amount of foam you get on a keg pour of beer is effected by a lot of variables - how clean the lines are, how cold the lines are, how long the lines are, the diameter of the lines, whether you're using beergas or co2, how old the beer is, if it's keg conditioned or force carbonated, how recently the keg was moved into refrigeration, how cold the beer itself is, if it's the first pour of the day or if the tap has been running frequently, the mechanical design of the faucet, the temperature, cleanliness, shape, and size of the glass it's going into, and more. It's really fiddly business, I can't see how a push button system could take everything into account and render less wastage than a human operator with a feel for the system. Draft systems are voodoo, ask me how I know.

Anyway companies typically have an unrealistic expectation of what draft wastage ought to be. I would advise any bar to expect something like 15% wastage at minimum on professional draft equipment, more if they're using bargain grade hardware anywhere in the system, but ownership doesn't want to hear that.

Because metering pumps cost more in the short term than custom code, boards, software stacks, and most importantly...consultants.