In a sense. a "stoic" expression would not have been out of place.
Or even an option to be upset about anything besides what Bioware thinks we should be upset about. On Thessia, I didn't think it out of place that Shepard be angry about what happened. But I wanted my Shep to be p*ssed at losing the beacon, not at Thessia's fall.
DAI at some points allows the Inquisitor to express a range of emotions: sadness, stoicism, anger, confusion, approval, etc. in ME3, mosdt of a conversation can go by with only one or two binary inputs from the player.
I agree that stoic wouldn't out of place, but it would be really difficult to convey in those scenes. Especially since the ME team wants to Shepard to react in those situations in a way that allows others to react as well. Besides, the Renegade overlaps with stoic at times.
As for the DAI comparison, it is because they are two different games with different goals. You are correct that DAI allows those emotional reactions chosen from the wheel, but as a consequence they are very momentary and even there the PC cannot choose to be sad about a specific element, but rather jst sad.. They are literally reactions to what is happening at that moment. By having Shepard be sad about the loss of Thessia, ME3 in turn forces the player to express that specific sorrow in some way, tells a story of how Shepard reacts to that emotional stimulant.
Neither of these approaches is wrong in anyway, it is just a different choice on how to approach the narrative and what is the PC conveying in those scenes.