Why does our body need more blood more quickly when we are feeling nervousness?
It has to do with fight or flight respond. Basically your body gets itself ready to respond. So your heat beat increases to have your muscles ready and fueled with energy.
Back when evolution was designing our bodies, the main thing we were nervous about was being eaten. The faster heart beat meant more oxygenated blood, which is better for flight or fight situations but not so much when it comes to taking a test