Because human emotion and they would be much more angry and take it out on her. Look at what terrible thing they already done.
and why human is doomed ? The girl said they keep them so the outside world don't know about these experiments. Which mean humanity is still exist and still kicking, with morality still high so the inhuman thing they do here have to be keep secrets.
Prolly a long while since you read this, but they were talking about how they were developing a cure.
And how come no plan for future is her punishment ???
The only reason one would ever punish is to prevent the same thing or something similar from happening again in the future. Only people unable to control (aka inept) the evolutionary developed punishing/revenge behaviours (which is there to instinctively make the species make itself a poor target for predators etc. so that any that tries won't try again - and eventually that gets adapted into their instinctive behaviour to avoid. Exactly why most animals instinctively fear snakes or brightly colored prey) would lash out without first rationally consider what future the disciplining will lead to.
Same reason justice systems are not there to punish criminals as a goal, but rather does that as a means to the actual goal (stabilize/safeguard society).
As such [since punishment is de-facto an act of planning for the future], if there is no hope for the future, you have no reason to ever implement punishment (besides instant gratification).
also, they do still do cresearch with the data they already obtained. They literally said so
They also said "but without the physical specimen... all of you, let's go" with a despairing face, before apathetically leaving (not even bothering to give the culprit a second glance). That's rather telling.