Maybe for the same reason total yards are used instead of yards per carry for running backs. The former, and similar total stats, were used for years as benchmarks. It is only in the last 10-20 years that new statistical measures have been created and the league hasn’t adopted their use as much as the fans have. The league still emphasizes the “old” stats as those are the basis for records.
But reading various sources, I’m seeing more and more emphasis put on EPA stats and the like. The points per drive is a useful measure but it’s hard to separate teams when the numbers and differences are so small.