Give me 1 good reason why. Its suppose to be about playing the game, not grinding the requirements for the various perks. And don't try talking crap about how you can play the game while levelling them. Perks generally start to become really useful by level 3, 4 brings no major changes and 5 is where you spawn with a weapon. If it spawned you with a lower tier weapon at earlier levels it wouldn't be such a problem.
Because starting with a weapon makes you exceptionally powerful. It saves you tons of cash and gives you extra firepower straight from the beginning. That's why level 5 perks are supposed to be something difficult to achieve, but with high rewards.
Remember, the game is supposed to be about survival. And part of the feeling of survival is starting out weak and then buying a larger arsenal as the more dangerous specimens show up. You should have no trouble having the weapons you need by the time you need them.
The only time a starting weapon really becomes necessary is on suicidal. But that's why it's called
suicidal. It's supposed to be unfairly difficult to play compared to the other difficulties, so having a level 5 perk balances that out.
On an unrelated note, I agree with Ihmhi, it would be great if the perk leveling encouraged good team play. Though considering the firebug is such a general killer, with the heavy enemies being the exceptions, they could probably stick to just damage.