Why is landing not usually a continuous descent?
The plane wouldn't be ascending during the descend sequence. It instead follows a stair-step sequence. The times where the plane remained level may have felt like ascending even though it wasn't. It is very hard to judge that in the passenger cabin.

It is much easier for the pilot to have a sequence of descends to a specific altitude alternating with keeping altitude until the next marker. It also provides a timeline during which he can handle his checklist. 

Doing a full continuous descend brings the danger of ending up too high and needing to re-intercept the glideslope from above. This needs more vertical speed and if something goes wrong can end up with the plane undershooting the runway in the worst case. Intercepting from below each time is much safer and makes that the airplane will never descend steeper than the glideslope and the horizontal sections allow for a margin to correct any minor mistakes.

It is also safer to change the configuration of the airplane (flaps/gear) while in level flight rather than during descend. Staying level for a bit also allows the plane to bleed off the airspeed gained during the descent.