just curious why there isnt any single leg movement included (like lunges or bulgarian squats). any specific reasoning behind it?
disclaimer: im not trying to discredit the routine in any way or change it at all, im just curious and asking.
I expect the reasons are:
No need, as the program works well without them
Having to do twice as many sets, each using much less weight than would be the case with bilateral, is not advantageous unless there is a specific thing being addressed.
Which for a more advanced lifter may well be the case, depending on the individual and his specific situation.
But isn’t usually the case when starting.
Lastly, I expect that there is intent to focus development of skills on the major lifts, rather than having learning ability diluted across a large number of exercises, let alone those that are extra-demanding of skill due to balance issues.
Of course, I’m not Rippetoe, and never read an explanation of his on it, but those are the reasons I would guess.
it’s a beginners routine. simplicity is it’s strongest point. that and the fact it works.
cheers for the straightforward replies 
Plus your squatting 3x a week and Doing Deadlifts at least once a week…
The focus of the book is on strength on the core lifts. Lunges are supplemental exercises for the lifts, but are meant for injury prevention and fixing limb imbalances.
They can be done in the program in place of squats for a day(wed for example). Not a bad idea if your looking for some extra recovery and prehab into your routine!!!