You are probably joking, in which case sorry to be obtuse with what is to follow. But this is why Vegas stays undefeated. People get ideas in their head of what "always" happens, and those ideas are so hard to change no matter how much evidence piles up.
Under the current iteration of the coaching staff (i.e. since the start of the 2017 season), Iowa has the 10th best record nationally ATS in games they are favored. Among just P5 teams, they are 4th best.
Free college football team ats trends and splits in simple, easy to read tables. This page tracks as favorite results.
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Over that same stretch, they have the 2nd best margin ATS as favorites (+8.2), trailing only Bowling Green at +10.6 (BG has only been favored in 5 games over that span, to Iowa's 30 games).
And we have just won consecutive games by 42 (favored by 5.5), 28 (favored by 3), and 20 (favored by 1). Not a lot of "letting them hang around" going on there.
You know what Iowa hasn't been goog at sine 2017? Beating the spread as an underdog (bottom 20 in the country). They are roughly middle of the pack (technically slightly below middle) at winning outright as an underdog.