They can't afford not to!
> I am happy to walk a while to get to the closest bus stop so I can get to the tube.
You're lucky that you have access to walkable pavements, busses, and a tube. That's a lot of privilege! I'd sure they'd be happy to use those as well! But many people don't have access to those things, and if they want to get anywhere they need to pay taxis.
If I want to get somewhere outside my village in less than a couple of hours I'd have to use a taxi or drive.
But if you go to for example some parts of the US, they literally don't even have pavements let alone busses, let alone tubes.
They can't even walk to their local shops in some cases. They're trapped without a car or taxi.
Yes, one of the reasons you mention is why I never accepted a job in the US as I would be stuck!
Lucky for you that this is an option. For many people less fortunate than yourself it is not an option and they rely on taxis such as Uber.