Well... exactly 5? No.
But, I can explain, sort of, how it happened. This isn't the exact thought process, but it will get the same point across.
Imagine I started with a system where you had 20 MaxSP, and abilities could cost any amount of SP: 1, 2, 3, etc, and you recovered 1 SP a turn.
But, then I wanted to do some SP restoration effects that were less than a turn's worth of SP, such as Vigorous's passive SP regen, or Bloodlust's SP recovery when hitting multiple targets.
The choices at that point are either nasty fractions, or inflating everything I had so far by 5, so that now there's room to play between "zero" and "one turn of SP".
Of course, having done that, I could also use that new room to have abilities that cost fractional parts of turns too, such as an 8 SP ability for "one and four-tenths a turn"... but I like that keeping it multiples of default SP recovery makes it easy to see how many turns it takes to get the SP back, and so far, I haven't needed that much granularity in terms of SP costs for abilities.