The UNSC vehicles are armored with Titanium alloy and polycarbonates structures. Warthog are no exceptions They are both lighter and more resilient than modern armored vehicles. According to the Wikis they weight 3 metric tons, and have a maximum speed of 75MPH. They're rather an arm platform than a proper combat vehicle, but given the guerilla oriented fighting style of the UNSC before the covenant war, it make sense; you don't make ultra specialized vehicles against insurectionnists. It's a waste of time, and money.

Also, it fit with the Spartan narrative; you don't need a ultimate commando in a conventionnal war, except as extras for specialized mission. If the Spartans II got so much funding in the first place, it's because in this particular war, they were the ultimate weapon.

Their capabilities being augmented dramatically lately in the war, by introduction of Mjolnir personal armor, but particularly for the Major, the bonding with Cortana, that ensured an optimal use of his tactical skills for maximal strategic effect.

Reach glassing is considered a tragedy, because at it was said, it's a loss of most of the Militaro Industrial Complex of the UNSC, but of most of it's top scientists and commanders. The Covenants essentially blown half the industrial potential of mankind, but most of it's elite.

The very very late implementation of "some" improvements on the Human gears, mostly "portable" MAC Gun on Warthogs, kind of relate to this, as with the diversion of much of the industrial ressources of the Sol System toward the construction of the UNSC Infinity, both in term of raw material and means of production, and nerds.

So, I kind of support the idea of an obsolete hardware, adapted to a previous type of conflict, but it must be said that the design was already at the top of it's potential as an industrial design.

Even against the covenants, except in large motorized confrontations, which covenants avoid in different, more or less radical manners, the type of warfare the UNSC practice against them is guerilla.

Since as soon they are in orbit, they virtually have the de facto air superiority, it also delay the need for mass implementation of specialized material.