I mean, that's the sliding timescale in a nutshell. We just accept the fact that the 15-year old Peter Parker who got bitten by the spider in 1962 is the same Peter Parker who's probably around 30 (something Marvel won't even care to admit!) in 2024. There are all kinds of potential in-universe explanations for this, but this is basically how it works.
More to the point,
why does Marvel have a sliding timescale? After all, if we were just dealing with episodic adventures with an unchanging status quo (like most Golden Age/early Silver Age DC stories), then there's no need for one - the characters are frozen in one particular age, and nothing ever changes so it doesn't matter what year it is. But Marvel has always had the passage of time - indeed, it was
defined by it. Characters experience things, and evolve. Time passes, at a slower rate than the real-world, but it passes.
That's what sets 616 apart from other Marvel continuities (and indeed, from DC, which does have a sliding timescale but also relies on periodic reboots).
It doesn't matter if Peter Parker got bitten by the spider in 1962 or 2009. It doesn't even matter
exactly how many years have passed since then (though its good to have a ballpark figure that most writers and editors can agree on). What's important is that a substantial amount of time
has passed, that Peter has experienced things and changed, and
continues to change.