Without T1 arty, T1 land battles would be solely about spamming tanks. T1 uberspam is already ubiquitous, reducing it to a pure numbers game would be pretty boring and even less about positioning and tactics.
And it makes more sense for the T3 arty to need to deploy: it's got a massive gun with huge recoil, also from a gameplay perspective T3 arty would be very OP if you could just patrol outside a base lobbing 3000 damage in every few seconds - as it is you need to support the T3 arty and take time to make it relatively secure.