The reason is likely, lots of mediocre QBs that play behind O lines that are just as bad as those QBs.
There has been a dearth of quality offensive lineman in the league for what seems like decades, the same can be said for the QB position, there are a lot of average and below average QBs out there, as well, there are a lot of coaches that aren’t particularly good at their craft. I mean face it, half or more of the teams in the league are pretty bad every year.
Now punters may be playing a part in this decline, but the bigger part, imo, is the above mentioned stuff.