At times, one does see, especially when they move around to get into the landing corridor that they seem to slow down considerably...yet as they get into the landing corridor one distinctly hears the engines revving up, gathering speed and that speed is still maintained when they land...

Why can't they slow down considerably before they land???

The landing speed and the take off (lift off) speeds are the lowest speeds in flight. It is universal for all fixed wing aeroplanes. Of course, some safety margin is added, lest the plane stalls (remember stall?)

You are going by the feeling of deceleration and acceleration you get while inside.

In cruise, the plane is clean. All lift augumenting devices like flaps and slats are retracted after take off and since no such thing is protruding out, it is called a clean configuration. Flaps are seen hanging at the rear edge of the wing while flaps droop down from the leading/front edge of the wing. You can see it when the plane lines up for take off.

These flaps and slats are used to increase the lift produced by the wing. They improve the camber (curviness) of the wing so that the desired lift is produced at lower speed. For the same lift to be produced if the wing was clean, then you would require much higher speed. Since you want to lift off at a low speed without consuming much runway, you always use flaps and slats.

Now, you were cruising at high speed in clean configuration. Approaching the destination, you commence descent by closing throttles and maintaining more or less same speed. As you come closer to runway, you start decelerating to your landing speed. So far, you are correct.

As you come even closer, you extend your landing gear (wheels), flaps and slats. Extension of these things add to the drag and enable to decelerate much faster. Since you are still descending, your throttles are closed. You use a much higher flap/slat setting (wing is much more cambered) for landing and these settings produce much higher drag.

As you are about 1000 ft above the runway, you have to reduce the rate of descent to about 600 to 800 ft/min. With all these things extended, when you reduce the rate of descent, the speed starts decelerating very fast. Therefore, you open throttles to arrest this drop in speed. Inside the aeroplane, you hear this throttle opening and assume that the pilot is accelerating. But that is wrong. Throttles are opened to only maintain the speed.

The jet aeroplanes are designed such that the throttles have to be opened to produce 85 to 90% of the max thrust of the engines in order to maintain the speed with these extended and descending at 600 to 800 ft/min. This is because, there is usually a time lag (10 secs or so) in all jet engines from the time you open full throttle and the engine produces full thrust.

Therefore, in case the pilot has to abort the landing and go around, this 10 sec time delay would be too much (easily a height loss of 100 ft, calculating as per that rate of descent). Therefore, the approach is configured in such a way that he makes it with his throttles already producing 85-90 percent max thrust. So in case of go around, the time delay in attaining max thrust is negligible.