Because this kind of AOB reading, as opposed to method 4, degrades rapidly over time, especially at shortish ranges or high speeds. If you spend 1 minute drawing the necessary lines and getting the AOB, it may not be the AOB anymore.
One of the best things you can do as a subsimmer (if you don't want to use U-jagd tools or the OLC gui which provides this in-game) is to build yourself a whiz wheel that provides instant AOB-target course solutions as well as other useful stuff.
provided by hitman
http://www.subsim.com/radioroom/show...06&postcount=1
and a handy manual for it by klh
http://www.subsim.com/radioroom/show...ight=Whizwheel