Detonation? Really? Maybe in very carefully controlled lab conditions where the powder is separated to each end. Norma supposedly did it ONCE and reported that it simply was not something that could happen without planning.
Why do people do a lot of things? 'Cause they consider it easier or better.
.357Mag make rounds just as accurate as .38 Spl at the same velocities, but not at the same charge weights. However, if you aren't a reloader, buying .38 Spl ammo is simply easier. Are there even are any .38 spl equivalent factory .357 Mag loads.
I do all my shooting in .357 Mag revolvers using .357 Mag cases, as I don't like cleaning each chamber of the "soot ring" left with .38 Spl cases in the longer chamber. Just as easy as loading .38 Spl.