is it that hard to understand nes games weren't hard as just a gimmick to extend the games length,

I guess the first thing I would say is that while nes games are a heterogenous bunch, I do agree that on average, they were harder than contemporary games.

That said, there were most likely multiple reasons why nes games were harder than the games we play today. One reason definitely was the need to slow progression due to size constraints. Games were miniscule in size then, and I really think it stretches plausibility in that context to say that difficulty wasn't used to extend play time.

But there were other reasons too, like expectations on the part of players and developers about how a game "should" play and feel. And over time developers have experimented enough that they are bound somewhat less rigidly by these expectations.

Ultimately, difficulty (or the absence of it) isn't intrinsically good or bad. There are great games I have played where it was very difficult to die, and games I have played for over a decade without beating and have died thousands of times at.

Saying a game "sucks because it's hard" is probably unfair, or at least incomplete. What matters is how the difficulty fits the rest of the game.