There’s no shortage of reasons. I’m not going to list them all and deprive everyone else of the opportunity to hit the nail on the head, but I will give you one I bet you didn’t think of:
Venture is not a general purpose mining ship. It is not designed to mine common or even uncommon ores, basic or moon. That’s what most Joe Alphas using Ventures are doing, and they’re wasting their time. If expedition frigs are released to Alphas, they’ll be doing the same, which is not what they’re designed to do, and they’d likewise be wasting their time, only in a more expensive fashion, and in a fashion more juicy for gankers.
Venture is a good ship for guerilla gas harvesting and mining the rarest gases/ores in hostile environments in a low-risk high-reward fashion. The “low-risk” is in relation to the cost of the ship and “high-reward” in relation to the cost/time/risk of importing materials from tradehubs when you live far away from them, especially in LS/NS/WH. I remember back when I used to launch Guerilla WH mining ops staged from HS where a 2.5m Tristan fit would locate our friendly neighborhood WH, enter it, find a belt, clear all the rats, and summon an armada of Ventures to mine ABC(D) ores (rare) so we can procure massive stockpiles (relative to our needs) of the rarest minerals in the game. A couple of hours of mining and would provide us with stockpiles of rare minerals that would last for months, whereas we’d continue to have chronic shortages of common minerals like tritanium and pyerite that demanded barges hitting up veldspar and scordite (or embarassing imports from Jita).
this is the proper use of a Venture. Alphas being given access to T2 mining frigs would only further encourage mining ores in HS/LS, which is not what they’re for and is activity we want to discourage on the basis of being passive (bad), dangerous (gankers), and unprofitable, and unfun. It would be a high-gank-risk low-income-reward zero-fun activity for Alphas to use Ventures or T2 mining friends in this fashion.
As for Alphas using T2 mining frigs in the proper fashion I described… upgrade to Omega.