I believe that it has to do with the speed of light in a vacuum. You see, you may be seeing the lasers hitting at a certain time, but they are in fact hitting well before then.
I sincerely doubt this is the reason. John Q. Average likely doesn't know a thing about this and certainly doesn't care.
I believe it's more likely to keep processing requirements low on this end, what with the potential for extreme fleet sizes with extreme damage outputs. I don't think it'd be very pretty for a low-ender to have to process some 60 bomber squadrons attacking a starbase if their hits were all calculated per impact.
The exceptions are abilities that affect units on a flat per second basis - such as hull repair or hull damage - where there's absolutely no complex math done. It's just X added to or subtracted from Y. This was probably done since they're the minority and greatly affect a ship's survivability/lethality.
