What is mining difficulty, really?

"Back in the day" when the mining difficulty was low, you could mine using your laptop and cpu. Today we can't because of the asics and mining difficulty.

So back in the day, say 2012, an asic s21 would have won every block. Today, it doesnt. And it isn't that the blocks are processed faster.

So I don't get it, what makes it harder? Is it amount of permutations? That has to be tied to transactions? So if the transactions stay flat but the hashes/s go up, shouldn't block time go down?

Yeah, I just don't understand difficulty. Is it an artificial constraint?