But in answer to the question "why are radial engines so hard to start", well that is because most old aircraft piston engines are essentially very crude machines. The conditions of mixture and timing etc of a piston engine have to be guessed at by the person starting them, or using their experience as to what is about right. There are no measurements to give direct feedback as to whether the person is getting this right, and only backfiring, exhaust fires and lack of ignition to indicate when they are getting it wrong! Also the carburettors, valve clearances and piston ring sealing are less than optimum until the engine has reached operating temperature and run for several minutes. So the cold, crude aircraft piston engine is a right difficult bugger.
Modern car engines have very sophisticated engine management, ignition and fuel control systems which analyse, adjust and optimise engine settings hundreds of times each second. They have fuel injection and knock sensors and exhaust analyser probes to optimise the mixture and ignition timing and they have hydraulic tappets to give near perfect valve clearance. All of this makes them extrememly reliable and dependable. I am old enough to remember the 'dawn chorus' of people in the street trying to start their crude, low tech cars in the winter in the 1970's. You had to know the knack of jiggling the throttle, and the choke (and in some cases the timing) to get the damn things going. I mean, they had a physical mechanical switch to operate the coil circuit to produce the ignition sparks, and the timing was adjusted with a vacuum device that physically moved the switch to advance or retard the ignition!! You had to be sat in the car so that you could operate all the variables just to get the engine started. Nowadays, You don't even have to be in the car - just turn the key to start the engine and the ECU does everything for us and to a much higher accuracy than we can. Modern electronics has made this possible.