If a car had only one gear setting, that would mean it needs to be able to start the car, travel at 100 mph, and climb hills all in one. That’s not realistically possible, or if it is it’s very inefficient. Heres a graph showing the power and speed limitations of the different gear ratios. As you can see the force at the wheels and road speed vary greatly with the gear ratio. So for most cases if you’re on a highway you’ll likely be at one of the top gears because you don’t need as much force and need speed, whereas starting a car and climbing a hill you’d need a lower one because physics. Now if your question is why don’t we have cards that do that for us, well we do. Standard automatic transmissions change the gear train automatically in different circumstances, there is at times a slight lag if you go from flat ground to a steep hill possibly but it catches on quickly. Continuous variable transmission also changes automatically but it does so using pulleys apparently, and you can basically get every size you need, and it does all that automatically for you. Some people don’t like CTV though because it doesn’t usually cause jerking movements or stuff that manual or automatics have with their presets, so some paces water down CTV and make it more limited so that it does cause some jerking (still less than other types I’m pretty sure) to make people happy which I find kind of funny. That being said, Europe doesn’t sell many automatics. Like a source I saw said 80% of cars in Europe sold are manual, vs the 3% in the US, so if you wanted to ditch manual and get automatic you could but it’s less likely to happen.