Balance. Pure and simple.
That million-g.p. character would - if it had any more brains than a shoe - spend some of that money on the best adventuring equipment available: warhorses; war dogs; field plate (if a warrior); every spell in the book (if an arcane caster); masterwork everything; hordes of retainers and henches; and - most important - any and all useful magic items it could find. All before ever theading out into the field.
And it would still have enough left over to buy a castle and the village next to it.
Meanwhile here comes Joe with his scale mail and shortsword because he can't afford splint and a longbow.
Now I don't usually give a flying fig about in-party balance but this is over the top even for me.
Okay, so it is only a problem for game if
1) The player only uses it for themselves, creating disparity in the party.
2) If they can or want to buy things that disrupt game balance. Yes, a player could try and hire 50 Veterans to fight all their battles for them. But, the potential to do something doesn't mean that has any interest for them. They might not want to sit back and have the DM describe how their kill squad went and won the fight again... but they might want to have that kill squad go and handle something else.
Would it be wrong for the party to use that gold to hire those mercenaries to go and deal with a lesser problem while they deal with a bigger one? I actually had a character drop a hefty amount of gold once to hire people to recover an NPC companion of ours... because we needed to delve in the opposite direction to prevent the villains from getting a superweapon that could be used to annihilate cities. And everyone was fine with that. We clearly had more important things to do, but we also didn't want to abandon our person.
Also, looking at your list.. a lot of that is of iffy value. Masterwork weapons don't do anything in this edition. A masterwork Greatsword is the exact same as a mundane greatsword. Warhorses are of debatable value, unless you are fighting in open fields. Even full plate armor doesn't turn you into something that can't be challenged by low-level play. It makes you harder to hit, sure, but with bounded accuracy a guy in plate can still be brought down by goblins and kobolds.
Some of this stuff is potentially problematic, but transferring gold into magic items or hordes of retainers is not easy to do, and if it would impact the game, that is when the DM starts asking what the goal is. But... there aren't rules hiring mercenaries. It is all DM fiat for the parts of this that are game-breaking.
Saving the world's all well and good but at the end of the day most characters IME want a payoff.
Yes, but they don't tend to get paid in gold. And also, they don't expect to get paid in gold, so it isn't an issue I've run into.
That's cool - until they turn around and sell 'em.
Has never once happened. Legit. And I've given (and made up on the spot do to insanity) plenty of cool things.