So here's the thing: while some people do think Elden Ring and Dark Souls games are hard, they're really not. From Software has more than a decade of experience making games that feel challenging, while at the same time bending over backwards to give players feedback and an excess of tools with which to succeed. And it's no surprise that after doing that for so long, Elden Ring is easily their most accessible game to date. They are masters at their craft.
The same can't be said for FF14; the game is just not very good at teaching people to play it, nor does it give players any variety of tools or methods with which to succeed. Credit where credit is due, most non-Ex/Sav fights have gotten pretty good at introducing boss abilities at a cadence where players can fail the first deployment of a new mechanic without wiping so they can learn from it on-the-go, and I really love that about FF14. But after a full decade, there's still nothing that teaches players how to effectively use the only tool they have to actually beat a fight: their kit.
Creativity is rewarded in Elden Ring. You can beat the same boss in wildly different ways depending on your build, the tools you use, and the way you approach the fight. FF14 offers none of this. FF14 asks you to dance a specific dance, and if you are at-level there are generally no alternative steps you can take. FF14 has right answers and wrong answers, and sometimes a continuum between them along which you can be more or less right. Creativity can help you find the solution, but it can't help you find a meaningfully different one.
In addition, Elden Ring lets you over-level. As an MMO, that's simply not an option that FF14 will ever be able to provide its players. I mean, you can wait a handful of updates until there's gear with a higher iLevel, but that's wildly different from taking an hour or so to grind some runes and giving the fight another go.
If you just have difficulty, but you don't offer players an accessible variety of ways to tackle challenges, your game will never be anywhere close to the success that Elden Ring is. Elden Ring is where it's at because the people who made it are experts at crafting games that give players every possible advantage so that they can overcome content that would otherwise be difficult. It would take a drastic pivot for FF14 to even approach Elden Ring's success in that arena.
(And beyond that, I'd say you can't really compare the sales numbers of an MMO with a non-MMO, as there are plenty of people who won't play an MMO just because it's an MMO, or they'll drop it because it's a really big ongoing commitment, or they can't afford to pay monthly for a game, etc. There are simply other factors that need to be taken into consideration to do a fair comparison.)