I don't know if most of them do. If I look around in the android market there are some great apps, that very likely do not make any money although they offer actual value to the customer.
I don't get it, why should apps be released for free?
Especially on Android I am surprised as to how many free(mium) apps are available, which could and should be paid apps, since they offer value to the user. Many apps however useful may not generate enough revenue by offering an ad-supported version of their app.
It's sad to see that the developer community is shooting itself in the foot by releasing so many apps for free/ad-supported. I am pretty sure that most ad-supported apps will not create a great revenue stream for the developer. Buyers simply get accustomed to this trend. See the divergence of revenue models between Android and iOS. I don't know if it's the market that speaks here, or maybe just a trend in developers not trusting in the value of their own product. All in all it seems that this trend of releasing apps for free does nobody any good, since it will stall innovation at some point.
On a side note, I would really like to see a comparison of how some similar apps perform which are either paid, freemium or donate. Maybe there is some threshold for when it makes sense to publish an app either freemium or paid.