Because otherwise, files can contain characters that aren’t on any of its lines.
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1...:
“3.206 Line
A sequence of zero or more non- <newline> characters plus a terminating <newline> character.”
That’s consistent with:
> echo test | wc
1 1 5
> echo -n test | wc
0 1 4
