Things get bad slowly and it can be hard to notice at first. When companies slightly shrink their products or change to cheaper ingredients, most people don't immediately notice. To an extent, it's like the boiling frog analogy. You slowly raise the prices and lower the service expectations and people don't notice as easily.
People also become dependent on things. Even when there are alternatives, people have habits and those habits are powerful.
Uber doesn't have a monopoly, but they are usually a duopoly (with Lyft). It isn't easy to compare prices and services between. I hate saying that because if you are trolling, you're going to say "it's easy, you just open both apps", but it's not easy since the pricing is always shifting and with drivers canceling or pushing you to cancel it becomes problematic.
Uber's brand and one's history with the service can leave one hesitant to believe it is now bad. Maybe a few times you think "oh, it's just a couple bad experiences." Historically, you have more positive experiences to balance it out. Many people will want to write off these bad experiences because they've talked up Uber a lot in the past. They don't want the cognitive dissonance of thinking "wow, I was an idiot to buy into their bait-and-switch and get all my friends to buy into it!" They'd rather believe that the service hasn't gotten that much worse and that they still like it.
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We see this kind of thing in a lot of areas. If Google Search is so bad, why are so many people using it so much that it's so profitable? Again, many of the same things: it becomes bad slowly over time and people don't notice it so much; people become dependent on things and use Google as a habit without even thinking about available alternatives - it's the default search in their browser just as Uber is the app they have installed; while Google doesn't have a monopoly, there aren't many alternatives and yea you could just run your search twice in two search engines, but people aren't going to do even seemingly "easy" things to compare or get better results; and Google has a brand associated with search quality that people have pushed forward over a couple decades and people don't like changing their beliefs around that.
We can't make truly free decisions. We don't have a situation where one could say, "If Uber isn't serving you well, you could just make your own transportation network that treated you better and use that." That's not so possible. We're often left without truly free choices because we live in a society where certain things exist and other things don't and we can't rebuild things overnight when something isn't great.
Lots of companies make lots of money while providing bad service because they're using their market position to gain leverage. If your logic is "so many people are using it, therefore it must be good," that really ignores how companies use their market position to leverage more profits because people don't really have choice.