Probably because it's not easy. Spaces as indent are lossy - converting spaces to tabs can lose contextual information (eg. when they are used for alignment).

Probably because it's not easy. Spaces as indent are lossy - converting spaces to tabs can lose contextual information (eg. when they are used for alignment). You can easily and losslessly do it the other way around, though, which leads me to do exactly what you say when working on a space-indented project: I code with tabs, I convert before committing. It's usually easier than dealing with spaces.