The reason is simple: You are not supposed to fall on these climbs, in any case. Falls will in most cases be very bad indeed, no matter where you clipped in. Your security line is basically made so that you don't fall 1000 meters, and so that the rescue team has an easier time finding you and cleaning away the mess. The "real" security comes from being able to grab the steel line (or the lanyard itself) for stabilizing yourself, and for having dedicated steps, ladders etc. so you don't have to rock climb, thus opening up those routes to non-climbers.
This is strictly different from sport climbing where falling is most times factored in and usually not a problem - those routes where a fall must be avoided at all cost do exist (i.e. only very few, brittle spots to push in a friend or something like that), but are the exception. While this kind climbing is built around making falls safe, via-ferrata-climbing is built around avoiding falls at all cost.
I treat the lanyard as insubstantial. If I have the feeling that I cannot go on without falling, I will turn back, even though I have the lanyard. It may give me a little psychological boost, so I go just a bit further than without, but it is absolutely a minor part. One reason for this is that you can fall only once with your lanyard, after that it is worse than useless. This means if you bring yourself beyond your capabilities, and survive your first fall, even without incurring substantial damage, you're in extreme danger afterwards, with no security at all anymore.
Finally, if those ferratas were made with enough bolts to clip into them, instead of the line, this would be a logistical nightmare - both when creating the line (a huge amount of effort to place more bolts, also for actually finding proper places to put them), and for clipping all the time. A long way between two bolts is a service to the user of the via ferrata, as it means not having to clip so often. If this frequency were significantly higher, people would just not clip in.