Pleasure is immediate, whereas happiness (or joy) reflects something more long-lasting and is often ineffable.
You can get used to pleasure because you can predict it and control it. Joy and wisdom are not gained so easily, and it's that challenge that creates meaning.
If you had a brain scan of pleasure and a brain scan of happiness, do you think they'd be identical?
I think you understand that unbridled pleasure eventually leads to jadedness, because you excluded drugs. The other forms of pure pleasure do the same thing but over a longer timescale. Yes, you'd have a truly amazing time at first. Why do a lot of rich people turn to drugs or gambling?
You can get used to pleasure because you can predict it and control it. Joy and wisdom are not gained so easily, and it's that challenge that creates meaning.