Manufacturers of gun parts have two choices: make parts that are designed to drop in or parts that are designed to be fit. Each have their benefits. A drop-in part is obviously more convenient for the customer. A to-be-fit part will generally be made a touch oversize on purpose. Why? Well, it might be to accommodate tolerances/variances in the existing guns. Or it might be to enable a closer fit than a drop-in part would permit. A drop-in part has to be sized to fit the smallest in-spec gun. For the largest in-spec gun, that's always going to be a little smaller than would fit/work. By starting on the larger end, a gunsmith can remove just enough material to get it to fit and work.
Does this extreme tightness of fit matter? On most gun parts, not much or nor at all. But if one were going to pick only ONE part on which a tight fit is beneficial, it would be a barrel, because slide-to-barrel fit is a substantial contributor to accuracy.
Could Sig make a true drop-in barrel? I'm sure they could. But would it have the same upper-end accuracy potential of a must-fit barrel? No, at least not for any guns that are not at the very low end of the size tolerance range. Sig appears to have disclosed in advance which approach they were taking. That approach seems especially rational for a threaded, extended barrel. Almost by definition, that's going onto a gun that will have a lot of money in it. Or on it.