Because logging in is baked into use of the software. You can't just store a fully functional offline version of Java Minecraft, at least not without mocking all the server responses it needs (not a meaningful option for most people) or the cracked version that pulls out account features (piracy, an option for slightly more people.)

Aah, OK, I thought by "you" you meant Microsoft.

Based on the downvotes you weren't alone. :)