Probably because most engines only have visuals which work due to TAA. UE4, idtech, frostbite, northlight, etc. have all designed many of their effects (ambient occlusion, reflections, transparency rendering, etc.) around stochastic accumulation and then cleaning that up with TAA. If you turn off TAA in these games, most of their effects works breaks. So there is really no alternative to TAA in many of these game engines, or if you choose to not use TAA (like turning it off in an ini file), then you have a dramatically worse looking game that no amount of MSAA or FXAA could save.