Your game has various sources of gear or whatever your unit of character advancement is (usually gear). You might get it from quests, crafting, events, PvP, single group dungeons, or raids. Of course, whatever sort of gameplay you favor is the one that should produce the best rewards or at least have a chance of eventually earning something comparable to the best. In games with raids, especially progressive raiding, raids usually produce the strongest gear. And I have always been pretty much okay with this, despite never being all that interested in online synchronized dance recitals.
Because what are you going to do with the best weapon in the game as a solo player? None of the solo content assumes that you are going to have an extra thousand DPS, so you will just blow through it even quicker. Of course, by the time you get the best weapon in the game (TV Tropes warning, happy Monday), you don’t need it, so it is even more of a cosmetic reward. You will probably enjoy solo content less if you have raid gear that trivializes it. You do not need raid gear unless you are raiding.
But I know we have some readers who do things with MMO content other than enjoy it, so perhaps you have your reasons.