Large amounts of free money are very unbalancing. Like anything else, balance is needed. That's part of why the downtime rules 1) don't involve large sums of money, and 2) are randomized. The other part is realism. It's unrealistic to always be 100% successful.
See, Lanefan gave me a whole list of things for it being unbalancing. A lot of it was stuff I doubt the majority of people would actually do. Such as hiring 50 mercenaries to win all the fights. Because that is boring for the players. A lot of the rest of it was things from previous editions, like masterwork armors and weapons, which in 5e have no mechanical effect.
Also, eventually, you ARE giving out those large sums of money. Remember that Troll hoard I mentioned? Single day making over 2,000 gold? With that amount of money the party could easily hire 50 mercenaries for a few days, it only costs 100 per day for that many if you are using the Skilled Hireling rules. Which, you won't do and you will charge more, if your players even attempt it, which they probably don't because it isn't worth the boredom of not being able to play the game.
Now maybe you have a different list, but large sums of money are part and parcel of the game, so they can't be that unbalancing.
Ah, but you said "free money" which brings me to a second point. What is free money? Is figuring out the average daily intake from working as a blacksmith with my passive blacksmithing proficiency somehow "more free" than looting the corpse of a mindflayer? I don't even know how that mindflayer died, it could have been far easier than the 8 hours of smithing work I put in. Generally, it seems that DMs have an aversion to "free" money, because it is safe. No chance of player death or maiming. Which circles me back to the top of my posts in this thread. 1) Pricing certain magical items at insanely high prices solely to prevent money exploits is bad design and 2) There is no need to fear "free" money, as long as your players have a motivation beyond profit. You can have a player or two whose motivation is profit, but much like Han Solo could have made far more money turning in Lea and Luke than fighting with them, the players should be able to easily recognize that participating the game they want to play is the most rewarding part.
Size. Utility. Ease of use. And more.
Great.
So an Airship that can hold 20 people and 1 ton of goods, with a crew of 10, that can fly at 8 mph costs 20,000 gold.
A Galley which can only work in the oceans or sea, with 80 crew and 150 tons of cargo space and moves at 4 mpg costs 30,000 gp.
These are exact prices.
A magic carpet that can carry 1 - 2 people (depending on size), 200 lbs to 800 lbs (depending on size), and can move from 20 mph to 54 mph (depending on size) costs... between 5,000 and 50,000 gp. Because it is impossible to put an exact price on it? It could be a 1/4 of the cost of an airship that can transport more people and goods or more than double the price.
Think about that for a second. That is like saying that an Ford Truck has a specific price, and a speed bike doesn't but it is anywhere from a fraction of the cost to double the cost of the Truck. That is nonsense. And they clearly felt comfortable pricing "flying anywhere in the world at double the speed" to the "slower and more limited, but far more cargo space" in the pricing between the Galley and the Airship. And, they didn't consult with the thousand and one details of everyone's individuals campaigns and whether or not that price would be appropriate or not.