It's kind of a combination of factors. On the one hand, the demand for apps has substantially outstripped the supply of good programmers, and the amount of money people are willing to spend on them also isn't enough to pay for really good programmers, and managers often can't tell whether programmers are very good anyway. Like someone said in the programming thread recently, the vast majority of people really aren't capable of becoming good programmers, and so the vast majority of programmers are mediocre and really only know how to repeat stuff. As a result, they do things they way they know how to do them and don't think about the specific needs of a project in the abstract, which is necessary for good design. So the average quality of programming really has declined, not because of any decline in individual ability, but because the ballooning demand for programming means that the bottom of the barrel gets scraped harder.
At the same time, though, the huge decline in costs of bandwidth and storage mean that there's no particular reason to be parsimonious with either. Most end users, overwhelmingly, really do like shiny UIs and don't know or care very much about how much space things are taking up, so they aren't going to pay extra for the effort of making a better but less spiffy app, so you can't afford to do it. Both of these factors contribute.
The alternative, at the end of the day, would be not having a Disney+ or Prime Video app at all, but only having those old passion projects made by obsessives, only accessible to those of us who are able to figure out how to use them. So I'd say the tradeoff is worth it for most people.