1; Ralts, I believe can be found on Route 34 *Johto region, right before Goldenrod City* after you beat the E4 and Lance.
2; For a few reasons:
- It'd be an easy way to level up your Pokemon. That's why the E4's Pokemon level up after a while in HGSS, too.
- Some facilities require that Pokemon be level 50 and if the Pokemon you enter is over that, it gets leveled down.
- It could give you an unfair advantage if you are in a Lv.50 vs Lv.50 battle that turns into Lv.51 vs Lv. 50
- If you are in the Battle Hall and you level up, the game would probably be forced to stall. In the Battle Hall, the level of your foe is based on the level of the Pokemon you entered and the Rank of the type you chose to battle. If your level were to change, then the formula for deciding the level of your foe would have to have to be altered, which would put an unnecessary strain on the software.