Oh, I think she was catatonic well before she even knew DSP. I also think that, DSP has provided her a better life. Again, you're coming at this from a normal person viewpoint. Khet isn't exactly normal. She's not like, Derich levels of brain damaged, but she's, to put it as politely as possible, simple. We might not know a lot of day to day details, but I think we have a pretty good picture of who Khet is. Hell, we even have a "what if" comparison with Jenna.
I also think you're overestimating how much DSP's life affects Khet. They barely spend any time with each other and don't seem particularly interested in what the other person does. He can't name any of the shows she watches or games she's playing. There were even some days off where he commented Khet stayed home. I'm not even joking when I say that I think the average detractor hears more about Phil complaining than she does just by virtue of watching Phil's stream. They're married in name only, otherwise, they're roommates. And not even particularly good ones at that.
I think, similar to Derich, she's as content as she can be in life because she really doesn't know any better. Instead of Twitter, she uses video games and Door Dash to fill the void. It sounds like she's also shown some signs of self awareness, hinting that she wants a better job. And if she was trying to achieve that, I might feel a little bad, but it doesn't seem like anything ever came of that and she returned to her content existence of playing video games and eating Door Dash.
To get a little real for a minute, people want to stay in a situation like that because, the alternative is actually worse. If I had to choose between being Khet or being Jenna, I'd beg for a bullet. But when I didn't get that and was forced to make a decision, I'd pick Khet's life. If Khet did leave Phil, what would really change? She'd have to settle for a smaller home, go back to working a minimum wage job and living on a potato diet. She'd still play video games in her spare time. And...that's probably it. She's not currently living the hermit lifestyle because Phil is forcing it on her, she's living it because it's what makes her content. Subsequently, it's what makes her so bland and boring from a detractor standpoint. Leanna, while far from perfect, at least did stuff, either making real appearances on camera or existing enough in the background where Phil could comment on what was happening. Khet just...sorta exists.