if matter can come from nothing, and if the first cell came from nothing, and then another and another and another, then why has that phenomenon never been observed to have occurred again?
Lots of possibilities why not.
Could be it is going on, but we think it's some kind of dark matter or dark energy.
And/or could be once a universe starts from nothing, any further creation of matter/energy from nothing forms separate universes, which we haven't yet identified.
Could be our universe didn't start from nothing. Could be it just started, and that there's no sense asking where it came from, because there was no time before it started and no sense asking where it came from because that would presuppose there was a time when nothing was there.