There's a couple points to it, and while it has to do with recovery, it also has to do with consistency. The chief problem that causes people to fail endeavors is a lack of consistency. While you may be able to keep track of whether or not you are on a lifting day or a rest day, your friends and family won't unless its always the same days and same times. That leads to double-booking, forces you to change gym times, etc. But let's also look at recovery.
If you are completely detrained, and have no hard physical activity (i.e. no sports) then technically you could lift every day. The problem is that you will only be able to keep that up for about a week or two before the weights get heavy enough that you need a day of rest in between.
Novices will need a day of rest to recover. This includes sufficient food/protein/rest/etc. So while you will be able to sustain the every other day schedule for longer than the daily schedule, there will come a time where you need slightly longer than that 48 hour cycle. That means with the typical Starting Strength schedule two of the workouts will be at less than full capacity. That 72 hour break (two days off) will allow you to fully recover at the beginning of the next week. It's that transition period where your body is getting closer to intermediate (requiring weekly gains instead of session to session gains), but still mostly novice where that extra day of rest comes in handy.
What this translates to is that you will be able to continue making linear gains from session to session for much longer than if you were to simply go every other day. As it is, you will only be able to pull of the Starting Strength program with linear gains for roughly 3-9 months (depending on different factors). After that, you need to transition to an intermediate program (weekly gains).
Instead of having to change the frequency of lifting every couple weeks to couple of months, it is best to have a schedule you can plan on. If you know every Monday you are going to the gym and you are going to lift a known weight for 3 sets of 5 reps it takes a lot of guesswork and "feelings" out of the equation. That translates to more successful beginners becoming intermediate lifters.
At the end of the day, success is what we are after. So while it would technically be a slightly slower pace than the body could handle from a completely detrained state, the difference really isn't enough to be worried about. It's better to have some patience and follow the plan as prescribed.