In the last few years, MLB left fielders have hit less productively than CF, RF, 1B, and DH. They've been slightly better than 3B.
So of the 9 hitting positions, they've been better than four of them and worse than four of them (although they are closer to 1B than they are to SS/C, so they aren't equidistant from the extremes).
CF and RF have more defensive responsibility than LF, so in a nutshell, we would expect LF to hit better. But they haven't been. It's true that LF is still a hitter's position, but honestly, teams don't tend to be hiding any defensive liabilities there anymore. Some still do (Willingham, Quentin, Holliday, etc), but others use LF to stash an extra CF capable player in the pursuit of a more injury robust roster (Brantley in CLE, Young Jr. in NYM, Marte in PIT, Eaton in ARI, Blanco in SF) or to break in young players of all OF molds (Yelich currently in LF, Des Jennings was last year, Trout gave way to Bourjos for a bit, Eaton currently playing LF in ARI).
There's also an apparent unwillingness to spend on left fielders. You see lots of teams choose to platoon cheaper guys there these days (OAK, TB, BOS, etc).
It's still a slugging position, sure, but to some teams it's not that simple anymore.