It's for the same reason that you can't say "These computers are without monitors." "Das sind" is the key. If you wanted to say "those computers", you would not be able to use "das", since das is singular. You can only use "das" for a plural meaning if using it alone with "sind".
Because that means something else than "These are computers without monitors".
In your sentence, there are some computers, and you are pointing to a particular group of them and talking about "these computers". And what you say about them is that they are without monitors.
In Duo's sentence, there are some objects which haven't been the focus of conversation yet. You are pointing to them and talking about "these" -- a bit like "these things or these objects". And what you say about them is that they are computers without monitors.
Oh, in computer science there are plenty of scenarios that you'd want to run a headless system (or rather, a system without a monitor attached).
To do things on said system, you'd network in from another computer. Various protocols have been used--these days it is usually a mixture of SSH and Git that get used to interact with headless systems. Though SOMETIMES you use an IRC chat room to interact with them--but that is usually an indication access is a bit on the nebulous side of things.
In this case, it is neither "these computers" nor "those computers"; it is "these are computers without monitors". "Das sind" can be translated "these are" or "those are", but "these computers" would be "diese Computer". You cannot say "das Computer sind" because "das" is singular when modifying a noun directly and "sind" is plural. That would be like saying "that computer are" in English. However, "das" can be used as plural when alone with "sind", so "das sind Computer" meaning "these/those are computers" is acceptable.
I am under the impression that there is not a major distinction in German between whether you translate "das sind" as "these are" or "those are", because it seems to be used when the relative location to the speaker is not very important. You just cannot translate "das sind" literally as "that are", because English does not permit you to do so. German uses other words if relative location does matter in context.
For more information about "das sind" and similarly used phrases, I recommend reading through this older discussion thread: https://www.duolingo.com/comment/2399665