Because she's so oblivious to how they feel she doesn't think they care about her.
She's just happy to see them all and if it had been up to her she would have already left~
She doesn't really know they're being/thinking like that or that her words give a double meaning for them. In her mind she's just spending time with the boys, saving/helping as much charas as she can & making the most out of the 1yr she has left to spend in the castle before her ''sickness'' goes away.
XDD both ends don't know that they're misunderstanding each other.. i think if she says something like she'll be fine and all it's just gonna make em more sad and think she's trying to act strong for them not to worry.
i feel a little cringe, why don't she just tell them that she will live??? it's painful to watch hotties worry about her all the time （/TДT)/