The development of banking begins to mark the outer edge of what would usually be considered 'fantasy' vs. the cold, hard reality of the modern period. Tolkein certainly never mentioned banking. Glenda the Good Witch did not have her funds in an institution earning interest. Xanth had no consortiums that I can remember. When people conceive of fantasy role play, banking is mostly off the radar.
(radar doesn't exist either, but that is for a much later post)
I have for some years now encouraged my players to make use of banks, to no avail. I have pointed out that with the credit advanced by a bank, they could set about building the castle in the middle of their fief, they could preserve their coin against attack and they could arrange payments to persons in other cities to buy difficult-to-find products for them. They could even make arrangements to have ready funds waiting for them in such and such a location once they arrived, if they chose not to carry lumbering pounds of gold with them as they traveled.
To date, I have not had one player use a bank in any way. I presume they do not trust me. I presume they feel they will have to pay through the nose in order to obtain all this safety and convenience. I have never indicated that they would - in fact, to the contrary. I have also attempted to reassure them about the reliability of banks, that they had no reason to feel concerned. Besides, if there is a certainty that the bank would fail, why not borrow the money with the expection of never having to pay it back?
I am convinced that players, as a matter of course, do not trust banks on principle. I am further convinced that they would view taking advantage of the existence of banks as a sort of cheating. I shall try an example, hoping that the video game I quote is popular enough.
If you have played SimCity 4, you know that the way to make your citizens use a parking garage and a subway is to remove all the roads between the city and the industrial park you've built. It makes no sense that any people anywhere would allow a mayor to zone a distant industrial region, connect it up with a monorail and subway and provide no roads whatsoever for people to get there, but you can in the game and sure enough, people will use your subway system. It's pretty good for your transit budget.
But if you are like me, you won't do it. The game may allow it, but it feels wrong and it feels like cheating. Just because the game designers are stupid enough to make it possible, doesn't mean I think that's a rational way to build a city.
There are power gamers in the world, of course, who see it only as a game and who will respond to the above argument with catcalls of stupidity and shooting myself in the foot and so on ... but I get a kick out of seeing my cities progress in the game and I get a personal feeling of satisfaction knowing I built them up without using a programmer's game glitch. It is the way I am.
For a lot of D&D players, I think that building up a castle is an important esoteric part of the game, and on some level they'd feel cheap and even a bit ripped off if when the castle was up and functioning, they had to admit to anyone that in fact their equity on said castle was a mere 17% ... but that they hoped it would be paid off in fifteen or twenty years. There's something very creepy about that, something that smacks much too much of reality, to have any place in D&D even if the DM allows it. Most of us with house payments or car payments or credit card payments don't want D&D infiltrated with the least little bit of that mindset. In general, I think most players would rather had three orcs pop out of the bushes and get away clean with the 347 gold pieces they had in three backpacks than to know the money was safe and sound in a bank.
Particularly of late, banks do not have what might be called any status of 'fun.' If, at the gaming table, players could confidently cease to think of banks altogether, I think they could be happy.
But I suppose I have enough fondness for more modern fantasy games that I don't feel quite so bad about working out with the mythical banker how my agent in a city four thousand miles away is being sure to send me a bag of ostrich feathers once a month. I need those feathers for my research, they are terribly expensive here, I can never get enough, and having the feathers arrive on spec is a great weight off my back.
Heh heh ... feathers ... weight ...