Because After Effects is a compositing and motion graphics app and not a NLE. NLE's use entirely different mechanisms to playback and render video.
If you want Photoshop (unlimited layering ability) and unlimited combinations of mathematical functions to perform on video then, with any compositing program, you have to deal with loading the layers of pixels into memory, performing complex calculations, and spitting out the results. If you want to lay three clips in sequence then drop the clips into the timeline, select them all, and then use the keyframe assistant to sequence the layers.
If that's all you are going to do, use Vegas, but if you're going to combine other computer generated imagry, other video, or graphics elements into those shots with more finesse than a simple alpha channel key or blending mode, then learn to use After Effects. Once you catch on to the idea of layered composits (like a Photoshop expert, not hobbiest but real expert) then After Effects is intuitive and easy to use. You will get results that are impossible to achieve in a NLE. If the best compositing you require is akin to the keying you see in a local news broadcast then stick with your NLE. It will do just fine for most video projects.
Master carpentry required to build fine cabinetry takes more skill and more time than framing a wall for sheetrock and spray texture. The art of fine compositing and motion graphics are no different. The only way to go faster with today's technology is to spend significantly more money on software and hardware.
What Rick said. This is a typical "I use a plasma torch for my omelettes" or "I use a chainsaw for dental surgery" kind of thing. Really. You are completely misunderstanding what AE is supposed to do and how therefore its underlying principles and paradigms work and rather than learning and adpating, you want the program to be molded to your needs. This simply cannot work. You really need to sit down and learn the basics. Your journey starts here: Getting started with After Effects. And I know myself that this was less than subtle, so no need to make a fuss of it. I'm out of this thread, anyway....
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