That's an interesting question Germanicus......Anyone have an answer or theory ?
I think it maybe this: From memory a century of triarii would have 80 men equipped with 80 thrusting (shock) spears and a sword. Like any good hoplite. Across a legion (2x Triarii century's per cohort = 160 shock spears; multiply by 10 for cohorts (I know I'm fudging it for the 1-st Cohort) = 1,600 shock spears. Re-equip them as pila/sword infantry and the numbers work out at 3,200 missiles carried by the triarii, who would be carrying two pila each. These would be in addition to the 6,400 missiles carried by the hastatii and principes. Also velites carried 5 missiles each.
The main reason I think is that triarii equipped with thrusting spears are a defensive unit in essence. While being re-equipped as missile/sword troops makes them an offensive unit in essence. Giving the legion commander three sticks to hit the enemy with: The hastata. The Principes AND the Triarii. After all it's hard to be a live barbarian with an extra 3,200 pila sticking in you.