Australia's National Rugby League (NRL) is played in accordance with the Rugby League International Federation laws, modulo some minor changes outlined in official documents from the Australian Rugby League (ARL).
The RLIF law regarding knock-on states, with emphasis added, that
[i]f, after knocking-on accidentally, the player knocking-on regains or kicks the ball before it touches the ground, a goal post, cross bar or an opponent, then play shall be allowed to proceed.
Otherwise play shall stop and a scrum shall be formed except after the fifth play-the-ball.
This passage is quite clear: a knock-on requires play to stop.
But in the Glossary Of Terms of those laws, we are provided with the definition of Advantage:
allowing the advantage means allowing play to proceed if it is to the advantage of the team which has not committed an offence or infringement.
In the case that the team recovering the ball has not committed an offence or infringement, and depending on the circumstances, continuing to play on may be to their advantage.
Finally, under the Notes section of laws regarding Match Officials, we are given a clear indication that the referee (and they alone) may decide what advantage is, and whether it has been obtained. They must also allow advantage only when the non-offending team both has an "adequate opportunity" for an advantage and has attempted to use it.
The former explains why play is not necessarily stopped for a scrum: the player gaining the ball may have advantage from playing on. The latter explains why players will immediately try to regather the ball and make a territorial gain from that position: they can only have advantage if they try to make use of a potential advantage.
In the worst case, they are brought back for the scrum, which by custom and by law, almost assuredly gives them possession. In the best case, they make great gains in territory or are able to score a try. Hence, both players and referees have incentive and ability under the laws to play on, not needing to stop play first.
The advantage law applies to all phases of play, but where a team infringes in a strong tactical position the advantage should be allowed only if the ball goes immediately into the possession of the non-offending team.
The Referee is the sole judge of what constitutes an advantage be it tactical or territorial. An infringement is not ‘negated’ simply because the ball touches or is touched by an opponent.
The opponent must have adequate opportunity to take advantage and endeavour to do so before play is allowed to proceed.