Expansion tanks aren't code required in many areas including mine. Instead they appear to be an "as needed" thing. I've never had need of one before in any of the homes I've inhabited in various states. That is, I've never had a system that didn't soak up the thermal expansion somehow without the expansion tank. But now I do, so I had one installed.
Just guessing, but on most homes connected to municipal system if you installed a test gauge you would probably find the thermal exp. pressure spikes exceed 80-100 psig. These should probably have thermal expansion tanks, but most often don't. Some can relieve back to the supply, but new higher system pressure water pumps/towers and checkvalves might prevent that...tightening of the overall network and toilet fill valves/etc. is likely to increase the need for these expansion tanks in the future.