That's a very good question. There was a good explanation in the L-Jetronic manual I had for my '75 Opel Ascona... both of which are long gone, and my recollection of the reason is sketchy at best. Remember that for every rev of the crankshaft, only 2 cylinders will fire. I think the actual duration of the injector solenoid opening is about 3 milliseconds. At 6000 rpm, 1 rev takes 10 milliseconds, and at idle 1 rev will be about 100 milliseconds. During one crank rev there will always be two intake valves open for some fraction of the rev - maybe 100 degrees each - so air will nearly always be flowing into some cylinder. Remember also that the amount of fuel per 'squirt' isn't much (roughly 14 pounds of air per pound of fuel), and hopefully with good injectors that tiny amount of fuel is atomized quite well. Even at idle (100 msec per rev) the amount of time that an intake valve is *not* open will be quite small (would need to look a a cam profile to confirm this), only a fraction of that 100 msec. That isn't much time at all for the brief quirt of atomized fuel to settle down anywhere.

I think there would be a practical limit to this fire-all-at-once approach, and it would be at very low engine speeds. Think about the huge stationary diesels used on ships - the ones with con rods 6 feet long - they turn at maybe 100 rpm. Now if they were spark ignition engines, I think the fuel injection pulses would have to be times to the intake valve opening, since each rev would take over one full second.

Just my $0.02 worth.