In my mind, tornadoes usually appear in flat areas with some wooden houses, but not places like cities with many tall buildings. It is especially rare to see a tornado destroying a series of skyscrapers. Is this true? If so, what is the reason?
It is especially rare to see a tornado destroying a series of skyscrapers.
Yes, it's rare, but not for the reasons you think. The reason that tornados striking downtown areas is rare is that cities are but a tiny fraction of the land area in places where tornados most often occur, and downtown areas are but a tiny fraction of the land area in those cities.
That said, downtown St. Louis, Missouri (the center of tornado alley) has been hit by tornados six times, and downtown Shrevesport, Louisiana (the center of Dixie alley), five times. The list of tornados that have hit the downtown areas of large cities goes on and on and on.
I think you need some statistics to bolster your argument. It could be that there aren't so many big cities in America's 'tornado alloy', so the footprint of a super-cell storm has a low probability of crossing tall buildings. Check out tornado alley on Google Earth.
There are still unanswered questions, such as why some supercell's develop tornados, whilst others do not. Also, the question of the mechanism by which tornados dissipate their energy has not been entirely sorted out. One can say, however, that tornado development requires two conditions: strong thermal convection, and little or no low-level wind shear - such as might occur around large buildings. So the optimum conditions for tornado development are large flat open plains in which warm and cold fronts are converging.