Because nobody collects the data. That's the only reason.
A publisher, in theory, aggregates total sales data quarterly for royalty payments. But this is pretty approximate, because there's a constant inflow of bookstore returns and outflow of bookstore orders. Authors don't, as ORION says, get very specific sales information.
And when it comes to classic titles, there are likely several different publishers who don't share information.
The reason we get movie info and CD sales info in the US, at least, is that there are industry organizations that collect that data (or who commission someone to collect it). There isn't a book-industry equivalent in the US of the Motion Picture Association of America or the Recording Industry Association of America.
In the US, the different best-seller lists are compiled according to different criteria. In general, the compiling organizations select what they feel is a statistically representative sample of booksellers and extrapolate from that.
This article gives a (somewhat dated) overview of how different best-seller lists work, as does
this one.
