Material things do not account for why they continue to exist from moment to moment. They do not exist “essentially” or necessarily because of what they are. Since everything that exists in the universe is of this kind of thing - each may or may not exist at any time - then it is possible for everything in the universe to not exist.
Big Bang cosmology actually demonstrates that the universe (and everything in it) came into existence 13.7 billion years ago.
In order to properly explain why things which might not exist actually do exist, it is necessary to posit an Uncaused Cause which exists by its very nature necessarily, i.e., it exists by the very nature of what it is, and in that way alone properly or sufficiently explains the existence of everything else. It cannot BUT exist. It’s essence is “to exist,” or it exists necessarily.
Aquinas and others have suggested that God fits this description. Ipsum ess subsistens. God is the necessity of Existence or Being itself from whom all contingent things come to be.
In short, material reality does not, of itself, explain why it does exist, so some other SUFFICIENT reason must account for the existence of material reality - presuming, of course, that explanation is required.
It is possible to claim that no explanation is required. In which case, Aquinas’s arguments are not compelling to such a person. The problem is this position undermines a logical or reasonable view of existence by claiming things just don’t require explanation.