- We say that pilots must use the maximum length of the runway. Therefore, why don't we touch down on or near the threshold lines as our aiming point?
No. Using maximum length of the runway just follows from more general rule to have as much margin of error as possible. But errors can happen in both directions. Landing long is more common, so it makes sense to leave the largest margin at the end of the runway, but landing short also happens, so some margin should be left at the start too. The 1000 ft is considered reasonable margin.
Note that for small aircraft, some advocate using the middle part of the runway and leaving about as much margin at both ends. However the PAPI and ILS can't be adjusted for each aircraft, so they are installed at the standard 1,000 ft past threshold mark.
Besides, pilots don't have to use full length of the runway. There is also LAHSO.