Because it is highly, HIGHLY unlikely that it will manage to output 450 watts of power, especially once you PC starts to heat up. A 550w is a minimum for almost ANY build, other than very low power builds.
If you use a very good quality 450 that would mean that the psu under load would need output constantly 100%. That is very taxing for a good quality psu. You should allow at least 15% of breathing room for your psu. Also quality 450 w is hard to find.
The evga g2 might be overkill but the modular psu are things of beauty.
Super Flower, FSP and of course seasonic are some names of quality.