It all depends on what constitutes “might be chometz”. There are different degrees of possibility, and you can’t lump them together.
At this point, it’s considered more of a minhag than a chumra, but based on a time when there was a possibility, which meikar hadin we don’t need to be concerned about, which nevertheless some were machmir on. Some never took the chumra to the full extent, and relied on certain lenient factors which would not be acceptable for axreal issur.
Now, some have the minhag to be machmir even though we grind the wheat better and have thinner matzah, and they keep the minhag as their ancestors kept it when there was a (remote and not halachically binding but real) possility of chometz.