First, if your models are outnumbered, it is easy for them to get trapped. Normally if the enemy wins a combat your model needs to move an inch or so away. If it is physically impossible for your models to move, your models count as trapped and the enemy can strike twice! So even though you have a higher fight value than most models, if you have one bad round of rolling your model is good as dead.
Second, there are certain game modes where you are trying to get as many models onto the objective as possible. So if you are playing against a normal sized army (around 30-40 models) your 16 models just can't compete.
Third, when your model count reaches below half, you count as broken. Basically every model has to take a courage test or run. Heroes can call standfasts, but occasionally it will fail and your army will run off. Read the LOTR main rulebook for more details. Basically you only need to lose 8 models to get broken! Compare that to Goblins, who require 30 models to die to get broken.
Also, after the initial charge, your cavalry loses effectiveness, because if your cav get charged they dont benefit from their mount and count as normal soldiers. In my experience Cavalry is best used to support infantry. If you really want all cav, you should look into warg riders. Cheap and effective.
WHY DO LOTR BOXES HAVE 50% LESS MODELS FOR 30% LESS?