Because they were never designed to be watched in marathon form. I usually watch a few episodes of things back to back, so I've become familiar with the skip bar, but in the context of television, it makes perfect sense.
You take ~20 minutes, space it out with ten minutes of ads, or ~40 minutes with 20 minutes of ads, and then you start shows on the hour or the half hour, and people, by seeing the intro, understand what the show is, who the main characters are, and what it might be about. They want people to watch the show, and they don't know if people will have a guide available to work out what it is, so it gives them an idea. You'll notice that more formulaic shows have very similar openings. You can easily tell crime or law procedurals by their openings. More story based shows often back their openings with summaries.
Of course, the trend doesn't carry over well to audiences using boxed sets, watching 3 or 4 episodes at a time, because those intros get old when repeated. But they're still made for TV, so they still do it. Be grateful that there are some shorter ones now. Watching the intro for Star Trek more than once an evening is an annoyance I will not abide.