I don't know the Why of having 5000 kits,,,,,,,,but, I do know the Why of having 550
You get to 550 aircraft kits because kits and decals are OOP immediately when they are released, it just takes a while for all the places the kits get dispersed to to sell out
So, if a Tophatters Phantom F-4B gets released,,,,,,they only last until sometime before the next F-4B is released by that same company,,,,,and the next time might be VF-84 Jolly Rogers instead
Then, later on, when you have VF-102 and VF-84 built,,,,and you need VF-14,,,,,,,those kits in that VF-14 release are only at places that buy collections and re-sell them
This only has to happen about 10 times before you learn to just go ahead and buy kits when you see them, instead of paying collector's prices later on when you really need them
You also end up with "non mainstream" kits that are bought to supply one or two weapons or detail,,,,,and if you don't throw them out afterwards, they sit in the stash, because no one wants that kit with the missiles picked out of it (and nope, it is not okay to tell me that I can't buy Hasegawa second generation Phantom kits just to pick out the 2 travel pods and the outer wing pylons for use on "better" kits)
I never remember who it is,,,,,,,but, there was a guy with a Shipping Container full of model kits as his stash,,,,,I think he is here on FSM
Rex