I'll have to ask my players sometime if 5E is "too easy". Considering that last game I had an encounter where the barbarian and fighter were both knocked unconscious at one time or other and the other characters were beaten bloodied and bruised. According to the XP calculator I use (it ignores numbers of opponents) it was supposed to be a high-medium encounter.
Some of it was just luck, my dice were hot and theirs were cold.
Or in the game I played in where we tend to only have a handful of fights between long rests and my cleric had to use revivify and mass heal or we were looking at running away or a TPK. As it was, it was still close. It was intended to be a tough boss fight against one of the last of our major foes so it was appropriate.
The published mods may be on the easy side, but there is a pretty vast difference between effectiveness of parties. If they were not on the easy side, many novice parties would be wiped out on a regular basis. I DM two different groups. Both are fun, but the challenges I throw at one group to challenge them would wipe out the second group and vice versa. Also remember that the guidelines are designed for a party of 4, a party of 5 or 6 can handle quite a bit more.
The fact that some DMs have to turn up the knob a little bit for your group is not surprising, I don't know how you could design a game where it wasn't the case.
Difficulty is set by the DM.