Because you are searching for a regexp rather than a substring:
>echo "D:\0.0" | findstr /I "D:\0.0"
gives nothing either. You want to add the literal flag /l
>echo "D:\0.0" | findstr /I /l "d:\0.0"
gives:
"D:\0.0"
If you want to use the regexp, you need to escape the dot
>echo "D:\0.0" | findstr /I "d:\0\.0"