In the past versions (FSX and below) we could pre-select or at least look at the options in the ATC coms menu (such as altitude or approach changes, nearby airports freqs, etc.) while ATC was actively giving instructions - to us or other a/c. In MSFS we have to wait for ATC to stop talking before we can make a selection or look at the sub-menu items. This is annoying and inefficient. Hopefully its on the fixing soon list ?!
I don’t like that either, especially when I have to wait to reply to them as I count the seconds before they rag on me for not replying in time.
My GUESS is that they can’t change it because the app probably has to process each communication with the pilot one at a time as to not confuse it. Otherwise, you could select to increase or decrease your altitude several times before allowing the app to answer (not that anyone would have a reason to do that). Or maybe decide to ask for clearance to land then change to some other unrelated communication.
Like I said, that’s just a guess though.