We definitely do, we just don't want to pay to string up wire thousands of miles, let alone superconducting wire. The difference between keeping 98% of the power that goes through a wire, or 100%, isn't the reason we don't do it. To quantify it further, the current US grid loses 5% to transmission losses which is just less than a cent per kWh.

Most power is generated in a centralized way anyways because it's much more efficient that way. The 'dregs' aren't connected because putting up the wire costs far more than the extra power yields. A few percentage points more efficient won't change the economics, especially if the wire is (a) lead and (b) dramatically more expensive.

3.5% per 1000km is respectfully, basically nothing. You'd get 85% of the power out of a line from SF to NY.

I'm not saying there aren't use cases for room temperature superconductors, I'm saying this is not one that's going to be top of the list.