The most popular academic theory (IIRC) is that "um" and "uh" are conversational placeholders that say, "don't talk, I'm not finished speaking yet". Which obviously serves no purpose in a monologue.

To me they just indicate lack of confidence on the part of the speaker.

There's a correlation between speaking with confidence and bullshitting / corner cutting. Hard, nuanced questions require more thinking time to produce a nuanced answer. But a bullshitter will just confidently answer subtly wrong stuff. But they won't say "uh"! Is that really better?