So in World of Warcraft, once I hit a certain (and relatively low) threshold of gold, I was basically able to use the addon TradeSkillMaster and my oodles of free capital to move in on whatever market I wanted and take it over without having to give up on any other markets. I couldn't completely own it because people were willing to throw their time away farming mats as if that makes decent gold, but pretty much I could guarantee I was the one getting the most out of the market and part of that was making sure I deliberately did things to reduce what other people could get out of it. It wasn't just competition, but salting the earth behind me every morning. In FFXIV, while I don't have the gil to do that anyway right now, I'd be prevented from doing that by the cap on retainers. Since I can only have 40 auctions up at a time and not literally thousands, I have to choose a few things that will be the best use of my slot and go with those, rather than simply taking over everything because I can. I have to imagine that's the basic idea behind the limitation.
Also, basic economic forces keep people from simply selling 40 items max, namely competition. There is not unlimited demand. If everyone attempted to sell the same thing, the market would quickly be beyond saturated and prices would crash. Naturally that would discourage people from selling that thing, while some other thing would be relatively expensive, and they'd move on to that. Over time, we see people disperse to selling a great many things based on their own resources and preferences. This is a very basic idea of it, but it's essentially the fundamentals of economy and specialization.
