This scene, as written here, cannot and should not be taken literally. It’s ludicrous to imagine that all of Judea was queueing up for a single guy to give ‘em a dip in the Jordan. Logistically, no. Even if he had a Roman legion in place to enforce the order, round everyone up and guide them to a single spot on the Jordan, this is a non-starter because historically the Jews had responded so violently at the idea of simply being counted thirty years earlier.
Politically, no. Judean society at that time had splintered into diametrically opposed political groups, some of whom quite violently enforced their opinions. The very notion that all of them would agree to do something, even on order from the highest authority, is far outside the bounds of realism. This is clearly a literary device, like setting up a joke about people by having them walk into a bar. It’s a metaphor for something else, a reference to a shared experience.
Ancient Egyptians felt strongly that getting their affairs in order prior to their demise included gaining forgiveness for any errors against the gods and the ancestors they may have committed in their lives. This idea seemed to carry over into Hebrew culture, such that a contemporary of John’s might have sought out forgiveness of sins when they knew death would be near. John’s act of baptizing ‘all of Judea’ is a prophetic act, one that presumes the death of ‘all of Judea’, and calls on them to put their affairs in order. Here we see they all were there, washing away the sin of their rebellion in preparation for certain doom.
The setting of this activity, in the Jordan River, is not particularly glamorous. In fact, it is very far from being glamorous or urbane or sacred – they are out in the sticks, away from the urban areas, outside of civilization. That this is the “wilderness” is reinforced in the next verse, but the point is that the sacred purification baths that were detailed in Leviticus were located in sacred places, in private. Not outside in front of everyone in the middle of some salty river like what was going on here. This wasn’t absolution, this was public, group humiliation.