how is using lighter expensive bike using fancy 0.5% less friction gearboxes better for your health? If you wanted health benefits you would be using the heaviest shittiest bike possible for more resistance.
Aside from that, the joy of riding is what gets me out. Many cyclists choose tough hill climbs for added fitness gains and challenge. There is a good kind of difficulty (steep grade somewhere picturesque) and a bad kind (unresponsive bike that doesn't roll well)
I don't usually go to the middle of nowhere, most often I'm going through a bunch of towns. In the worst case I can shove the bus on a bike to get to the next town.
Time and time again I have found that failure happens even with the best of components, and having standard components means you can find replacements more easily. Having a steel bike means the village welding expert in a random town in rural China can fix the frame, or fix your broken rack. Having standard spokes, tire, etc. means the LBS in Burmese village (who unlike American LBS, are actually reasonably priced) will have parts that work with your bike. Having standard brake pads and cables means you can get them anywhere when they wear down.
Additional point: shitty bikes usually have really terrible geometries and will fuck up your health if you decide to use them for sport. If your shitty bike calls itself an 'MTB' and you take it onto a trail, it is very likely for it to actually break if you catch even a tiny jump (let's say 20-30 cm root) because the welds are terrible, and the metal is brittle.
But also, a higher performance bike will help you go further & faster. It doesn't mean you will end up exerting less effort, it just means that you will go faster and you'll keep putting in the same effort. With this time saving you can try new routes, feel more comfortable going further or bike packing with higher quality gear. It wouldn't change how long I ride for, but it increases the options for where to go.
Yes there are diminishing gains as with anything, but an entry level bike is still going to be below the Shimano 105 point of performance vs cost.