He has though.
58.2 xGF% and 66.7 GF% at 5v5. Both elite.
His 5v5 line and PP units have been generating shots and chances at his normal rate.
His 5v5 line and PP units have been generating goals at his normal rate.
He individually is getting his normal amount of shots, chances, and expected goal generation in both game states.
He's generating career high rebounds.
He's just picking up points at an abnormal career low rate (59% IPP, for a player who has been 70-80% his whole career) - which still leaves him producing at an elite level.
This is likely due to a number of reasons - injuries, normal fluctuations in puck luck (also resulting in a career low shooting percentage), a lack of free/easy points in OT/empty net situations, and the insane defensive responsibility that Matthews has had to take on to allow Nylander to do his thing, and jump up to get odd man rushes and breakaways - which is one reason Matthews is also the top forward in the league in blocked shots, and according to a broadcast a week or so ago, "defensive plays".
There's been a lot of goals where he had an important impact on the play but didn't get the credit.
Unfortunately, some people only seem to be able to appreciate his impact when he's potting 60 goals...