I think you are underestimating the ability it takes to be a positive in a playoff series. Against the sixers it may not, probably wont matter since they are so dyssfunctional right now. But in a tough playoff series weakness get exposes. Kanters lack of ability to defend in space, Romeo's complete lack of shooting, RWIII's fouling and defensive mistakes. Ojeles lack of ability to do anything off the dribble, GW lack of size and rebounding. The Celtics have a lot of guys who do one or two things well, then have serious weaknesses. Those guys tend to get played off the floor in later round playoff series.
Exactly.
Smart opponents -- which definitely includes the Raptors even if it doesn't include the Sixers -- can and will exploit those flaws to the point that certain players become unplayable.
It's true that Kanter was the starting center for a playoff team last year. Then the Blazers faced an opponent in the WCF that knew how to exploit Kanter, and his weaknesses became a huge problem.
Why does this not happen to opponents with similarly unplayable players? Why is this only a problem the Celtics seem to be facing?
because this is celtics strong. it is who we are and it is what we do. 
Yeah, honestly, lol.
Bucks -> Marvin Williams, RoLo & Sterling Brown. All very limited players.
Clips -> Reggie Jackson and JaMychal Green are both very limited players.
Heat -> Iggy, Olynyk & Derrick Jones are all very limited.
The Raps don't really fit into the description, but they have the most flawed star power out of any contending team.
Almost all those guys are at least arguably better than the Celtics bench players. JaMychael, Olynyk, Williams are all good shooters who hold up well positionally defensively. Iggy is a good defender, passer, cutter. Derrick Jones Junior is incredibly athletic and great at finishing inside. Any one of thes guys would be the Celtics 7th man right now.
Robin Lopez and Sterling brown meanwhile dont actually play all that much for Mil, Brown was a DNP and Lopez played 2 minutes today. So ya, they are limited, and as a result they dont play in the playoffs. Reggie Jackson meanwhile played today, but only because Beverley is still working himself back into playing shape and only went 20 minutes in game one. But even he shoots 39 from 3 on volume this year with a 2.5 to 1 A:T ratio and decent shot creation, and he's like the 9th man.
So ya, again the theme here is guys who generally hold up on one end while giving you one (or more) plus skills on the other. Its not entirely clear ANY of the Celtics reserves outside of Smart do that. All the guys you listed are veterans with much longer track records as well then RWIII, GW, Semi, Langford. Wanamaker kind of fits that bill.
And again, I'm not saying the C's bench is doomed to failure. A lot of these guys are young, maybe somebody steps up. Totally possible. Just have to see it.