At the beginning of the Halo series, humanity had weapons that made the Covenant crap their pants. The standard UNSC pistol could be used as a freakin' anti-air weapon! The rocket launcher obliterated everything nearby, and the vehicles were completely indestructible.
Meanwhile, the Covenant were equiping their highest ranking Elites with fuzzy blue things that did nothing but make the humans laugh, as they used bullets to kill enemies rather than fists.
Then, the UNSC had a sudden urge to "balance" their equipment. They made it so the pistol couldn't kill a grunt, let alone a Covenant ship. They also made it so the vehicles were basicly fast-moving death traps that the enemy could now hijack. The only improvements were that the rocket laucher now fired rockets that could lock on to vehicles, and the UNSC finally saved up enough money to get every marine a driver's license.
At the same time, the Covenant had upgraded their blue fuzzy things into something that resembled a sword, which could now allow the user to fly 50 feet towards any target they wanted. They also developed a weapon similar to the UNSC SPNKR, except the user could fire approx. 1 bajillion rounds before reloading.
The UNSC then developed a weapon similar to the original stardard-issue pistol, but instead of making it standard issue, it was reserved for maniacs who enjoy getting launched from an orbital station into a city for a living.
The Covenant then realized that marines were using the plasma pistol and BR55 together to kill shielded Covenant easier, so they did the only logical thing and made the plasma pistol completely useless.
In order to combat Banshees in the absence of the pistol, the UNSC designed Hornets, a slower moving, hard to operate vehicle that was only semi-competent at killing infantry units.
Not wanting to fall behind on the manufacturing of over-powered weapons that make no sense, the Covenant designed a seven foot tall hammer that could destroy any enemy with one hit. This weapon was of course down graded when the Covenant realized it was useful.
Only with the help of the Elites with their retarded glowy things that somehow enabled their users to fly towards enemies, was humanity able to survive.