That’s a good point, but fortunately it’s absolutely possible to write a parser game with a conversation system where you let the player choose from a menu of dialogue options.
The technical details depend on the authoring system which you would use, but it’s possible in all of the popular ones, as far as I know.
You can have a “TALK TO” command or similar, which will drop the players into a choice-based conversation, where different options (selected by typing a number, for example) lead to different branches of dialogue and so on, just like in other video games.
It can be seen as a slight drawback that the players are taken out of the free parser input mode into the menu-mode, where they might feel that the task has turned into methodically going through all the options like a lawnmower.
The advantages, of course, are that you can implement the exact flow and back-and-forth of the dialogue, and that the players will never be at a loss how to proceed.
Another option would be to have a traditional “ASK/TELL” system which works with topic keywords, but to combine that with topic suggestions to give the conversation a certain flow:
>ASK JIM ABOUT ALIBI
"Where were you at 6 o'clock?"
Jim starts to sweat. "Uh, I was in the library, spending some time with Goethe. Alice was there, too."
(You could: ask him about Goethe, ask him about Alice, tell him about Alice's statement)
Further reading:
@emshort and @mathbrush have written these articles which go into more detail:
Edited to add: Choosing a Conversation System from the TADS3 Technical Manual (but not TADS specific)
Recommendations:
Emily Short has given a lot of thought to the modeling of conversation, so you should definitely check out her games.
Eric Eve’s games usually also have smoothly-implemented dialogue mechanics.
Some suggestions:
More recommendations
These lists might be helpful:
Games tagged with “conversation”, sorted by highest rated first