Amber can't leave Becky for numerous reasons that aren't completely obvious at first. Let's get the most obvious out of the way: Becky is the perfect chauffeur, food-fetcher, ass-wiper, and moral support (Mean-muggin', insult-spewing Becky is the best version of herself. But she don't talk to Rafe that much y'all, as she suspiciously pointed out in today's vid

). Amber has it made right now - low effort income, low effort relationship.
There are more...
subtle things that make it hard to leave though, one being mobility. Amber can't fit into a car comfortably. She can't even walk for long stretches. How the hell is she going be transported all the way out of bumfuck Kentucky, which she so desperately wants? It's not like she can hop on a plane, and she has numerous pets to boot. How would they get transported?
Then there's the drama bomb issue. Becky had no qualms talking shit about her ex's big, dirty stained pantaloons on a public forum with Eric. Don't think Amber wouldn't fall victim to that either. I don't think Becky would instigate this, but if Eric had some good-good and posted something in passing or in a video, I think she'd partake. Especially if Amber did her dirty. Becky is clearly all about loyalty (even she can't let Rafe go for good, after all) and if Amber's trifling ass dropped her like a hot potato for someone new - especially if they produced more Destiny-tier content with her - she'd be fuming.
That said, watching Amber and Becky try to have any chemistry makes me sad, especially after watching today's video. Amber treats Becky like a child showing off her crayon drawings and not like the adult woman she should be and she has no problem putting this dynamic on display. Functional relationships need to have an element of encouraging and bringing the best out of each other, not holding someone back or making them look like a fool. I think Becky knows what's up in regards to their relationship too and won't leave right now be it because of depression or monetary gain or maybe some misplaced attempt to genuinely try and help Amber (which she fails at miserably).
They'll be together for a while longer, but it's going to be a journey even more repetitive and frustrating than Amber's so-called weight loss journey.