Because it's a very debt-based system, policymakers don't want deflation, and want a slight buffer against deflation, and hence 2% is an agreement for them.
MMT is very different than top to bottom QE. The environment we had from 2008 to 2019 was very top-down QE. MMT instead would be direct spending into the economy, including into middle and working class most likely.
I do think inflation trends upwards in next 5 years; subject to huge fiscal stimulus.
Primary dealers get liquidity by selling Treasuries. Reserves are very liquid, although the fashionable narrative is that they are not. They allow banks to buy more Treasuries, for instance. Bank holdings of Treasuries are at all-time highs; they're not selling existing Treasuries; they are serving as pass-through entities to buy Treasuries from the Treasury and sell to the Fed.