Because how would things exist otherwise? It all has to start from somewhere.... at least until proven otherwise. Like I said, humans don't have this knowledge yet, making it kind of pointless even discussing it.
I think that the only way we can tackle the "beginning" of matter or energy is when we can "end" it. Once we find a way to do that, we're golden.
Until then we're stuck postulating about infinity or using circular reasoning.
"Where did all the matter in the universe come from?"
"It all expanded out in a large explosion in the big bang"
"And where did all THAT come from?"
"From the previous universe's collapse"
"And where did all THAT come from?"
etc.
"Where did all the matter in the universe come from?"
"God made it"
"And who made God?"
a)"God"
b)"God has always existed" (much like matter)