What's to wonder about? People write software based on their
understanding of conventions and not a pedantic commitment to the
wording of the standard. Apparently, unlike various other names found in
the standard library, the standard integer types don't present enough
surprises when used in this way for people to change what they're doing.
Personally, I always use the qualified name (e.g., std::int32_t) or an
alias. I'm partial to defining i8, i16, i32, i64, and so on. What's your
preference?