Disclaimer; I don't know these "some people", I can only guess towards their reasons.
My guess: Because fluorine is so reactive that it does not exist... Joking aside, difluorine viciously reacts with whatever, even noble gases. however due to it being a gas and forming stable bonds, usually passivates stuff, it doesn't... explode. (well, mix it with a reactive gas, say hydrogen, or an element that does not passivate enough and you will experience its reactivity when you get covered with the remainder of it immediately following the explosion. Take care.) You just can't in a reasonable manner show its reactivity as well as you can with Cs. Cs comes as a soft metal (under oil, mind you), it does not "passivate", it will react with most everything, dissolve in everything and melt and cover everything else, this is usually followed promptly by intense fires and/or explosions. It seems more reactive, because it kinda is. It reacts wildly with more stuff than fluourine does.
Of course, I still hold fluorine to be the most reactive, just carefully prepare ClF3 and see if there are any limits to what it can incinerate violently. There are none.