They aren't. When fully healthy they have been a borderline playoff team for 4 straight years. Hopefully they get stung by the injury bee again.
Discuss, considering the vast pages other forumers have invested disproving that last season in particular:
1. There win loss hardly changes within seasons due to injuries
2. They hardly suffered more injuries than is the norm for an NBA regular season
3. They have hardly had stable clear thinking management till this off season.
I've already explained this at length to folks understating the catastrophic injuries the Nets faced last season, but to sum it up in short...
In Mid-December last year, the Nets began an improbable 12 game win streak spurred in large part by the shockingly rapid development of rookie Rondae Hollis-Jefferson and the blazing hot shooting streak of Jarret Jack. This resulted in the up-start Nets surprisingly finishing with 42 wins, narrowly making the playoffs and causing the Boston Celtics to select 15th in the 2016 NBA Draft.
But of course, you didn't see any of this... because Rondae Hollis-Jefferson's tragic injury in early December (despite the player already
proving to be a defensive specialist and leading the team in both defensive and offensive rating), caused a fracture in the space time continuum and a backwards splinter universe resulting in a butterfly snowball effect in which RHJ's rapid development never happened, Jarret Jack ended up getting injured, Joe Johnson found himself cut, the Nets shockingly plummeted to a bottom 5 record, the Oklahoma City Thunder choked away a 3-1 lead in the WCF, the Golden State Warriors choked away a 3-1 lead in the Finals, Kevin Durant improbably joined the Warriors, the Indians choked away a 3-1 lead to the Cubs in the World Series, and Hilary Clinton choked away a 3-1 lead in the Presidential election to Donald Trump.
As a result of the dimensional rift, many of us are now living in this cartoon alternate reality (Earth 3.1) in which Trump is President, Durant is a Warrior, The Cavs and Cubs are Champions and the Nets didn't make the playoffs for the 4th straight year. Some of us who practice multi-dimensional astral projection, commonly known as "the see-ers", still see mirrored glimpses of our true reality in our lucid dreams and are conscious to the deja-vu-esque signs of our splinter existence. For instance, I can ascertain that in the original version of Earth 3, the popular children's book "BerenstEin Bears" was spelled with an "E" instead of the "BerenstAin Bears" (spelled with an "A") that exists in this Hollis Jefferson injury reality fracture.

Another popular theory as to why I'm aware of the true Brooklyn path is that when we fall asleep, our consciousness is capable of swapping places with the mirror versions of ourselves living in the splinter reality and while our memories of the previous reality SHOULD be replaced with our new existence, those of us programmed with reveries are capable of tapping into our memories of both dimensions. If you too remember growing up with BerenstEin Bears books, it's possible you are also experiencing the effects of this dimensional rift.
That's it, in short. I'm tired of explaining this for people. Simple stuff, really.