This is faulty logic. "People do it, therefore it's correct".
People do do situps, people also think that saturated fat is awful for you and that dietary fat = body fat.
Look at the mainstream emphasis on low fat foods, and the suggestions that eating low fat means you'll lose body fat, or that people STILL Think you can reduce bodyfat in localised areas.
Things like doing arm curls will reduce body fat on your arms, doing sit ups will reduce bodyfat around your middle, it's all nonsense.
Broscience is rife in weightlifting, all those anecdotes that have zero supporting evidence being spouted constantly.
Unless you've got very weak core muscles, sit ups (proper ones that don't use your hip flexors, most people barely even use their stomach muscles for situps, instead letting their hip flexors pull them) aren't going to do much at all.
Body weight exercises only go so far, which is why people do weights. The absolute best sort of things to do for core strength is an exercise that actually engages the need for your core to stabilise your body, so things like squats with a free bar.
A good squat session can demolish your core muscles so that you feel like jelly afterwards, despite the fact that you're not directly using them in the same was as a proper sit up.
A lot of training in a lot of areas is very antiquated, a lot of boxing coaches still refuse to accept that weight lifting is beneficial to boxing, and look at all those professional boxers with stick legs and no arse, despite the fact that thick legs and a thick arse contribute a lot to pulling a heavy punch.
