It's a lack of offensive identity.
This is a gross over simplification. I agree with your other comments.
We rely on too many guys who play minutes that don't produce. How is that offensive identity?
But let's say you have player X do most of this or that, what happens if he has an off night. Usually with others teams have guys who can step up. We don't have that capacity to any large extent and often the drop off from our starters to their replacements is borderline pathetic.
Check out the advanced stats on VORP ( Value over replacement player)
League average is defined as 0.0, meaning 0 points above average or below average. Because above-average players play more minutes, there are far more below-average players than above-average players in the league at any time. A value of +5.0 means the team is 5 points per 100 possessions better with the player on the floor than with average production from another player. (In the 2018-19 season, teams averaged around 100 possessions per 48 minute game.)
To give a sense of the scale:
+10.0 is an all-time season (think peak Jordan or LeBron)
+8.0 is an MVP season (think peak Dirk or peak Shaq)
+6.0 is an all-NBA season
+4.0 is in all-star consideration
+2.0 is a good starter
+0.0 is a decent starter or solid 6th man
-2.0 is a bench player (this is also defined as "replacement level")
Below -2.0 are many end-of-bench players
https://www.basketball-reference.com/teams/BOS/2021.htmlWe have 9 who do not even register positive and many are negative. Grant Williams plays often but is 18th on the team., Nesmith is 19th some of this is who they replace but some of it they are not that good as well.