Yeah, I discussed this with a friend after the movie. My No-Prize rationale for why they couldn't do this is that Ebony Maw was more than a match for Dr. Strange magically and he was subservient to Thanos, so I assume Thanos' power exceeds his, therefore if Strange tried to use a portal to chop Thanos' arm off it wouldn't be able to penetrate his skin because he's too tough.
Also, there's the fact that even with though at various points he was attacked by a fully enraged hulk plus Dr. Strange using every magical attack he could muster, the only thing a regular mortal did that even made a dent in him was Iron man's ridiculously advanced nanotechnology suit, and that only drew ONE drop of blood. New and improved Thor and his hyper-power axe did a lot of damage, but that didn't even do anything permanent, so I figured a portal wouldn't do anything but bounce off his skin.
Plus the whole encounter was based on Strange looking into the future to find the one course of action that could end with them victorious, so maybe he saw himself trying the old arm-chop-portal move and it failing.
Or maybe I am WAAAAY overthinking this...