Thank you and no offense taken.
5a) one important reason is that most if not all of my players are themselves (at least currently) well on the "homebrew adds more headaches than it's worth" side of the fence. In other words, they aren't pushing for homebrew solutions. Of course, this is at least partly due to the edition being rather new. If they are asked to make a new (4th?) character and WotC still haven't made more options available, perhaps they'll start asking for tweaks more.
5b) I find that I like the idea of running D&D "as is". I see a value in finding out whether the RAW classes can best challenges and complete adventures. Of course, now that I see how they run roughshod over the official content this reason is fading. On the other hand, if anything, I would want options that reduce PC power, not increase it. (We all still like options and flexibility, so any such solution would not include turning multiclassing and feats back off)
Do note that my reluctance is strictly for "player-initiated" homebrew. For instance, I had my Monk player find a headband that essentially gave him access to monk features four levels higher once a day. That is a homebrew item that directly affects classes. But the crucial difference is that it isn't something the player can count on, it's not something he can plan for when charbuilding.
Perhaps the most egregious case is if I let you know you would find a Gauntlet of Ogre Power in the end of the first dungeon you delve into, already before you create your character. This would then allow you to create a greatweapon strength-based build with Strength 8. You would certainly do alright using a finesse weapon up until you reach the bottom of that dungeon. This is perhaps the clearest example of what I
don't want, and it doesn't even involve any homebrew or 3PP at all.
5c) you forgot "basic distrust of 3PP borne out of the 3E era"
Then add a pinch of 3) and there you have it!