Sorry I'm behind as usual and in catch up mode so, forgive me if this is already been thoroughly discussed..
IMO the reason that first comes to mind in questioning possibly why was it that the backpack, fishing pole, etc were not just disposed of with the body is the fact that if this murder was as some have theorized in it being IN NO WAY PREMEDITATED, BUT RATHER A SITUATION THAT WENT SIDEWAYS AND RESULTED IN DYLAN'S DEATH..IMO in that scenario I can see possibilities of there having been an escalation of tension between the two that at the time it peaked and Dylan ended up dead that they were NOT IN THE HOME, but rather either en route to or from somewhere and while they were away from the home that it is then that the escalation peaked resulting in his death.
In such a scenario it would have Dylan separated from his belongings at the time of his death.. His belongings and fishing pole would be in the home with Dylan's death having taken place at a location away from the home..therefor you would have the possible hiding and disposing of the body immediately thereafter the death occurred .. Afterward in returning back to the home there would be the realization of all of his belongings and what and where would they be put, disposed of, or should they be left in the home..IMO a wide range of choices in what to do with his belongings and what choice would best suit the scenario that was going to be given as to how his son came to be "missing".. He left out with all his belongings?.. Or that he left out with nothing and his belongings were all in dad's home?.. Should some be taken and some left behind?..
As I said many many choices of what to do once the death had occurred away from the home and his body having immediately thereafter been disposed of or hidden..and then to arrive back home and realize the belongings were there and what was the best way to handle all of the belongings of Dylan's in dad's home.. IMO I actually can see how the fishing pole being removed came to possibly be in looking at a scenario such as these above where the death took place away from the home and that the issue of all of his belongings being in the home was an issue that came as an afterthought upon arriving home with his body already hidden..
In thinking about the various choices of what to do I can see his possibly thinking to throw in the fishing rod as only another afterthought in preparing on how the gone missing scenario was being conceived after the fact of Dylan's death and the body already hidden.. IMO I could see him thinking that the fishing pole would be a good explanation of where his son may have gone off "missing" to while he was out running errands..as well as it would also serve as a great diversion tactic of misleading investigators toward searching nearby water for his body in knowing that his body was not going to be located in those bodies of water..
IMO it also fits with the initial statements made early on by Mark in his stating he had from the get go been directing LE to search the lake due to the fact that Dylan's fishing pole was "missing" along with his son..in that scenario all misleading in knowing for certain his body would not be in those areas he was adamantly stating from the get go should be searched..
All jmo in looking at the possibilities of why Dylan's backpack, belongings, and the missing fishing pole were not found with or around him..JMO in giving scenarios of how or why this possibly could have happened *IF* Mark is responsible for Dylan's death.. Again MOO!