It is because it can be said without a great deal of thought, all you need to say is just "um", "er" or "ah", so easy. "Um", "er", and "ah" are what linguists call "fillers". "Fillers" help conversations continue smoothly.

Although we may not consciously realise it, in a two-person conversation, people speak by taking turns. When someone thinks it is their turn to talk, they do. Otherwise, they listen. A two-person conversation becomes like a tennis match. Inevitably there are short periods of silence as people pause to let the other person take over the speaking. But sometimes a speaker doesn’t want to give up their turn and instead wants a little extra time to think about what they’re going to say next. They use a “filler” to signal this.