Rules are established, and designed, to standardize and govern the general behavior of play. Play must adapt to the established rules, rather than try and continually adapt the standards to adapt to play.
The "end of the run" is the either the actual spot the runner's progress ended, or the actual spot where the runner lost possession of the ball. The runner has honestly earned whatever yardage actually gained, provided there has been no prior foul, to assist him, or his advance. (NFHS 2-41-9)
A fumble remains in the runner's "team" possession until it goes OOB, but with the addition of a foul by the team in possession, after the actual run physically ends (the spot where possession was lost) the yardage EARNED by that team reverts to the spot the runner lost possession (fumble).
Dealing with the loose ball (after the fumble) as a loose ball play for enforcement purposes would deprive the runner of whatever yardage he had honestly earned (without the assistance of any fouls) beyond the previous spot, which the Rules consider unfair or excessive.