Part of the difficulty is that I think that when most people say "steep learning curve", they aren't actually talking about a learning curve, but an amount needed to be learned to get anywhere curve. To phrase differently, they are not talking about X = time, Y = amount learned, but something closer to X = progress in game, Y = amount you need to learn to increment to the next X.
In particular, most "modern" games try to avoid having you need to know anything before you start; the expectation is that you will be drip-fed things in artificially-simplified scenarios. Graph "Game Progress" on X, and "New Things You Need To Know To Progress" on Y; the usual goal is that the line starts pretty close to 0, and climbs gradually (a shallow slope). An example might be a flight combat game where the first tutorial starts you out already in the air, with engine, flaps, radar, etc. already configured in sensible ways for where you are, and pointed in the general direction of some sort of simple drone target that doesn't shoot back; and the scenario ends with a win before you have to land.
DF would be more like one of the old Microprose simulators, where you're expected to read several dozen manual pages and install the keyboard overlay before being able to even get your plane off the deck into the air, and there are several difficult-to-digest walls of "you need to know new things to get further or Fun Happens" in your immediate future. E.g. you've managed to survive a season or two sort of OK, then winter comes, you have no unfrozen water and haven't figured out booze yet, and bad things can cross the ice to your fortress. Compare to managing to just about survive your first air combat encounter, and now your first carrier landing is with a damaged plane...
Graphed as above, "New Things..." per unit game progress increases much more rapidly in this sort of old-school gaming, a "steep curve". Modern games try very hard not to do that to their players, because they're likely to rage-quit and go away, and that's a problem because modern games tend to need you to keep playing to make money (subscription, DLC, expansions, in-game purchases, ad revenue, whatever).
This is a potentially important point I'd not really thought through before. When you paid your $50+ or so in 1994 ($80+ in today's dollars) for F-14 Fleet Defender, you had the install disks, the 123 page manual, the keyboard overlay, etc. and Microprose had your money. End of transaction; if you throw up your hands with the manual in your lap somewhere in the process of trying to figure out how to best switch to the RIO "back seat" cockpit, adjust the AWG-9 radar from PDSTT (Pulse Doppler Single-Target-Track) mode to a TWS (Track While Scan) mode and figure out how many bars and degrees you needed to reliably lock up a new target while still keeping a tactically useful scan rate and, you know, not crash the plane or get blown out of the sky in the meantime... at some level Microprose didn't care. They had your money, you couldn't get it back, it was up to you to learn the game or not.
The vast majority of modern games, in contrast, depend on a continued relationship between the player and the developer; whether it's a trial version to induce you to buy the game, or one of the many ways games are cheaper up front but extract money from you over the long term. If you walk away, the developers, studio, distributors, etc. loose some or all of their profit; it's so vitally important to all of them that you don't give up, that there is tremendous pressure to make the game easy to get into, potentially with considerable opportunity costs elsewhere that could have made a better game for experienced players.
This is why fans of more complex games have needed to turn to alternate funding models like Kickstarter, "Early Access", and direct patronage of developers. The old "up front" distribution model admittedly allowed for some real disasters to get out, but it also permitted some rare gems that the current "pay as you go" model makes difficult or impossible.
P.S.: I don't want to sound like I'm dissing F-14; on the contrary, it's one of the pinnacles of PC simulation gaming, in many ways not surpassed despite the passing of two decades. But in Authentic mode in full Campaign, it's as close to flying a real plane as you're likely to experience outside of a multi-million dollar simulator (or a couple of years in the military), for a plane that had a reputation as one of the most complex electronic suites flown in something ever expected to get into a dogfight. It's arguably the commercially released game that goes the furthest in the direction of simulation realism at the possible cost of being too far over the head of most potential players. In their own words, p.21: "If you're a pilot with a lot of flight sim time under your belt, this chapter is designed to allow you to begin play almost immediately. On the other hand, if you're new to flight simulations, bear with it. ... FLEET DEFENDER, like the F-14 it portrays, isn't always user-friendly (especially the portions dealing with radar). ... Just remember, the Navy spends years teaching its naval aviators the same information that FLEET DEFENDER tries to teach in hours (and days)."