The basic combat mechanics of FromSofts games are specifically blocking, parrying, rolling, stamina, and general movement, along with all the different forms of attacking, casting and item consumption. Most of the difficulty of FromSofts games comes from mastering these mechanics, and then learning every boss encounter individually.

To say the games are mechanically easy just seems like such a nonsensical take to me. Simple perhaps, but lots of simple things are especially difficult.

Most of the difficulty is in using the tools at the right times. This completely permeates the design of the games.

At first the game will show you an enemy with his back turned, and you will discover that this initiates a devastating backstab.

Miyazaki has shown you a new tool, but in doing so has also given himself a new tool for manipulating you: A few moments later, the game will show you an enemy with his back turned, and you'll probably mindlessly rush in for the rewarding backstab, only to be bum rushed by an enemy hidden behind a corner that kicks you off the cliff. You really don't stand a chance at reacting to this, and yet again meet see the familiar "You Died"-screen.

This is the dialogue I speak of, it's easier to point out in level design but very much also exists with in the combat system. Some players will probably miss this aspect and think it's just cruelty. Like, how could you have known? Well you absolutely could have known by being being more careful and looking around that corner before taking the bait.

The combat in general works like that as well. First you're shown enemies that are susceptible to one particular strategy, and then the next, you're shown enemies that are not. The difficulty isn't executing, but coming up with an appropriate strategy for the situation. And the games are very cleverly designed to shut down one-size-fits-all approaches to combat. There are no universal combos to memorize because they don't want you to merely go through the same motions, they want you to engage with what is happening on a more direct level.

What? You don't have to 'learn' any bosses. You can beat all bosses by just striking a few times and rolling away. You don't have to learn any patterns or cheeses or weaknesses. IMO that's not a fun way to play.

You make souls games out to be "memorize the trick" simulators. No, no, no. That's the unfun approach.

Dancer of the Boreal Valley would like a word with you.

While the strategy could be oversimplified to 'hit hit run', it misses the nuance of learning how to apply that strategy to a given boss. I ended up clocking over 600 hours on DS3, between personal challenge runs and invasions, and as the other comment suggests, your level (and gear, and how you distributed your stats) will have a big impact on how difficult a boss feels.

My first playthrough was an absolute slog, since I had very little idea what I was doing with combat, and had no idea how to optimize my character. On my 20th (or whatever) time, with a purposefully under-leveled character (Strength build), most bosses were one-and-done with the right game-sense and cheese tactics (Greatclub is great, and if you disagree, you are liable to be flattened).

If I'm in the zone, Gundyr is a minor roadblock between character creation and Firelink shrine. I think my record is around 2 minutes to activate the shrine bonfire?

Vordt? Gimmie that raw +3 (I think) broadsword with lighting resin, and he's dead meat at SL1[0] since his timings are very predictable after the 15th time killing him. Getting a raw +3 broadsword is the hardest part there.

Dancer, Pontiff, and the wrinkly pink screeching guy in the castle side area are still my hardest fights due to their patterns and movesets.

[0] SL1 = Soul Level 1, achievable when you start as the naked guy with a club and never level up. All other base classes have a starting SL greater than 1.

running up to bosses and smacking them to death really only works if you're way over leveled for the place you're in. And not even then, for many fights. A lot of fights you need to deal with additional enemies before the main boss, or even simultaneously. Ornstein and Smough, from DS1. Or Rom from Bloodborne. You definitely need a strategy, especially when that second phase hits.