Because he is showing his belief that banning guns will prevent such things and he's absolute in his belief!
And logically, it simply is NOT true.
I often imagine how half the country can demand that the other half give up its only defense against the Left’s tyranny. The fact that they all want to just ban guns by fiat proves that the do not give a fig about the democratic process, natural rights of self defense and the US Constitution.
Hey Beta, I'm a law abiding citizen, who served this country for 22 years. Do you really think taking MY guns would make the USA a safer place, that me giving them up will equal no more mass shootings?
I know the answer already. He knows it will have no effect.
HOWEVER, it might get him votes with the rabid Left, and leave me and people like me, vulnerable to their political edicts.
In the end, like anything else, it's about power. Power to force your will on others. It's the inner voice of every Leftist.