Where I live, if a drain pipe under your concrete basement floor changes direction, then the plumbing code requires that a clean out be installed at that change in direction so that each straight section of pipe can be cleared separately.
I really don't know the reason for this rule.
So, it's purely a guess, but it might be that there's a change in direction of a 1 1/2 inch drain pipe under the concrete at that spot.
I'd be inclined to use an electric impact wrench to remove that plug and put in a new one with antiseize compound on the threads. That way you'll stand a better chance of getting it out if and when you need to.