Earlier quoted context omitted.
I'm not sure of the exact details but taxis are different. Think of any big city taxi - NYC, London, etc. - you've ever gotten into. Bigger. Boxier. More trunk space, etc. Now consider GM's plant closing in the USA. Plants are configured to produce certain vehicles. Volume matters. Yes, cars and taxis are close, but for a number of reasons probably not as close as you think.
Until recently, the most common vehicle I saw being used as a taxi was a Ford Crown Victoria -- a large standard sedan with an aftermarket partition between the front and back seats.
The UK (and especially London and its famous purpose built "black cabs") is probably the biggest exception (the models are exported and can be found outside the UK, but I doubt that would be the case if TfL didn't have such stringent license regulations that simple conversions are not an option).