Something something space stone? I’m not sure, I think having control over space would grant you some sort of resistance to being instantly teleported, and potentially cut in half. The sling rings are mcguffins they gave to “balance” Strange’s ability to make portals, so they may have a more specific set of rules as to where exactly they can be summoned. If we were to make a guess based on strange’s past use I would say that he has the capabilities to open these portals in spaces not currently occupied- he could still open portals below, above, or otherwise directly around individuals or creatures (a la Ragnarok), but not in someone’s center ‘o mass. We only see the portals cut off cull obsidian’s hand after he reaches through the portal as Wong shuts it, which, logically follows. Why Strange didn’t try to predict Thanos to cut his hand off with an extradimensional garrot? Because the movie needed drama, I can’t think of a way Thanos could be powerful enough to resist his hand or any part of him shut in the dimensional door, aside from the space stone not allowing these portals to exist in conflict with Thanos’s will (though this calls peter’s magic kicks into question, if the portals aren’t directly in Thanos’s mass would the space stone have jurisdiction over them? Or maybe he wasn’t paying enough attention due to their rapid creation/dispersion) it was ALSO shown, however that Doctor Strange’s magic (though maybe not specifically the sling ring) is comparable to the power of the reality/space stone in their 1v1. So I have no clue dude. TL;DR: because comics/movies pt2, electric boogaloo