Valkyries. Pretty much the only reason.
Also, humanity was almost annihilated in those first months. The UN left all the areas too close to Breaches to hang and global population once again went into nosedive. Europe, North Africa and CONUS had rebuilt human populations in the years Post Impact and well, no longer.
Casualty and death rates for every branch of the infantile UN Armed Forces were horrendous, hundreds of First Gen Valkyries died in those first months too. Retaliation with the remaining intact nuclear weapons managed to break up the groups of conventional AG forces pouring fourth from the Major Breaches, at which point they stopped attacking and seemed content to buildup forces for the rest of 2057.
Types were also really rare, less than one thousand in total for the first year of the conflict were seen. They only entered 'mass production' in 2059, at which point all simulated models pointed to Gen Two Valkyries being unable to hold back the increasing number of Types and the eventual destruction of humanity. Armstrong Lunar base started building Arkships at this point.
Then in 2061, during the Battle of Britain, humanity found the Three Hundred and nearly completely wiped out Antagonist forces in Europe, culminating in the destruction of Uralsk Breach, the Eastern most AG Breach.
It was a very brief celebration as the First Type Zero, Indra, appeared after the destruction of Uralsk Breach and stalled the march westwards. Indra was initially thought to be one of a kind, so with the Three Hundred and increasing numbers of Second Gen Valks, humanity continued to push. When Abraxas cut apart Noah, Type Zeros were still just two of a kind and humanity doubled down on winning on Earth as fast as they could. When Aswang basically wiped out the Central African Arcologies in 2063, the realisation that this war would be long and bitter sank in and plans for institutionalisation of training for Valkyries formed.
