Because venturing out with an expensive DSLR wrapped around your neck on a daily basis will not only make you seem like a sinister pervert, it's also not practical - the difference between HEVC and ProRes has already been proven to be beneficial enough back when the 13 Pro launched, so that argument can be put to bed
Like saying why take your Airpods with you on your train journey when you can drag along a Japanese tube amplifier, because it's mental
Their point was about creating pro video. Smartphones will always be limited by their size - there's some crazy automated post-processing going on to improve the results, but they still have small lenses and small sensors which put a cap on the quality they can produce. If you're a pro videographer making pro videos you're going to use pro equipment - and despite the Apple putting 'pro' in the name of some models, iphones are not pro equipment.
And if you're just taking videos of your kid taking their first steps or your neat skateboard tricks so you can post them on instagram, nobody's going to tell the difference between a video you shot in HEVC compared to ProRes anyway.
Your Airpods analogy is backwards - you don't use Airpods as a PA system in a stadium because it's totally the wrong tool for that application. An iphone shooting in HEVC is the "Airpods on a train" in this example - and if you're doing something where quality is more important than convenience you get yourself a device that meets that use case.