I know she's in hiding. But can't she hide at her old house? And where is her husband? He's not dead, and I know that she didn't love him but I'd still expect them to be together. Lastly, by the ends of the books she not in hiding anymore, so if her house was originally unsafe, what to stop her going back now?
It appears that in the 7th book, Voldemort has taken over Malfoy Manor as the “headquarters” of the operation. So living at the Malfoys would be similar to living in Grimmauld Place like the Order of the Phoenix. Plus, it’s her sister’s house, so it’s not even like she’s staying with a stranger or a friend, it’s her own family. Also at this time the Malfoys are basically under house arrest, so she also may be acting as a kind of jailer as we see her following Narcissa around wherever she goes.
We also know from The Cursed Child that Bellatrix may have another reason for staying at Malfoy Manor:
We find out that she is pregnant with Voldemort’s child during most of book 7, and having a safe space to stay while with child is a plus.
"But can't she hide at her old house?"
You just answered your own question. It's her old house - literally the first place anyone searching for her would look.
Afterwards, as has been commented, she just follows Voldemort wherever he goes. She's utterly obsessed with him; I doubt she values any other human life except him (and potentially Narcissa).