Why it accumulates
Every task a system or a person ever needed to perform tends to leave behind the permission it required, because removing access nobody is currently using does not fix any visible problem — it only prevents a future one. Over time, the gap between what an identity actually uses and what it is permitted to do widens, quietly, without any single decision that looks wrong in isolation.
This is why access reviews so often turn up permissions nobody can explain: each one made sense the day it was granted, for a task that ended long ago.
Where the cost actually shows up
Standing access is invisible until the identity holding it is compromised, misused, or manipulated — at which point every permission it was ever granted becomes available to whoever now controls it, whether or not the current situation has anything to do with why the access was originally issued. The cost of over-provisioning is not paid at grant time; it is paid, all at once, at the worst possible moment.
Scoping access to the lifetime of a task instead avoids this by construction: access that expires when the task ends cannot be inherited by a later compromise of the same identity, because it is simply no longer there to inherit.