It seems that most are having trouble getting the general idea. If they don't get the general concept, they aren't going to grasp the specifics. Why don't we look at the questions at hand?

Q. "There must a reason why issit a NA engine gotto be high rev ?"

A. The answer to this question is that the premise of the question is false. High-RPM operation is not a requirement for a normally aspirated motor.

Q. "since RPM kills motor why issit there is still so many racing engine builder wants to build these kinda engine ?"

A. Because a racing engine only has to last until the end of the race. The specific reason why they build high-RPM N/A motors is because the rules limit the amount of engine displacement and power-adders. Without those limitations, they'd be using cubic inches, not "cubic dollars" to boost output.

Q. "After market Cam tends to have long duration n higher lift which shift the power band to a higher RPM and still many people buy . . . why ?"

A. Many people simply don't understand that the scalar number for peak horsepower is the least meaningful specification for street use. They buy parts that offer the highest horsepower numbers, not knowing that in order to produce that level of torque, and therefore horsepower at a high RPM, the cam is causing the motor to make a lot less at lower RPMs. It's a common beginner's mistake.

Q. "its like why people don't build a engine with the power band below 8000 RPM so they need not rev so high everytime which in the end made the engine last longer ?"

A. The reason why people who know what they're doing build high-RPM motors is because of rules that don't allow them to build horsepower more cheaply and easily. Their competition will do whatever they can to get the most out of a motor within that set of rules, and so must they if they want to be competitive. People who aren't bound by any such rules do it because they're stupid, they want to emulate real racers, they think it's "cool" etc.

That takes care of the questions. Now, getting back to my point, adding RPM to increase output has its limits. The faster a motor turns, the less time there is for each of the 4 cycles in an Otto-cycle motor. 2 of those cycles involves moving gasses into and out of the cylinder. Less time means less gas movement. Less gas movement means less output. So obviously there is a limit after which adding RPM produces less output, not more.

I think the people who have posted the horsepower formula fail to understand that torque is not a static number. A motor that produces 150 lb-ft of torque at 5000 RPM will not make anywhere near 150 lb-ft at 8000 RPM. Therefore the assumption that more RPM == more HP is false. You can tune a motor to move the torque curve up and down the RPM range, but that's a function of tuning, not RPM.