Why would you make statements that aren't true? Have you actually tried all longer cables? Perhaps some longer cables don't work and some do. The ones I have charge my new iPad fully, which no issues.
I use these:
Cable length would matter slightly as the longer cable does add additional resistance which represents power loss, but I doubt it is that much. The problem is more likely in the connectors on either end or the connections made along the length. A cheap cable will have poor connections and poor electrical connections would certainly result in charging problems. Or any poorly made connection anywhere along the length of the charging cable. Cheap is cheap. But it is certainly possible to buy longer cables that are both inexpensive and will charge an iPad 3.