I have an answer to this.

Mathematics (as in, the universal system of relationships between numbers, structures, sets, etc) is immutable and unconditionally present. If "nothing" existed, anywhere, the laws of mathematics would still be exactly as true as they are now. As far as I can tell, mathematics is the only thing that has this property. So, that made me ask myself "what is existence?" Basically, what is special about our reality that makes it able to exist outside of mathematics? Is there anything special? I would argue that no, there isn't.

If we were simply mathematical structures, then there is nothing stopping us from perceiving the universe in the same way we do now. We would perceive and think in the same way, governed by a specific set of mathematical restrictions (physics). My proposition is that the universe itself is a mathematical structure and only exists because it is one possible mathematically well-defined structure. Physics overall, and specifically quantum mechanics, kind of give me this impression, too. Everything in the universe seems to follow well-defined mathematical patterns. If the many-worlds interpretation of quantum mechanics is correct, then the universe as we perceive it is just one point on a line that goes through an infinite number of potential universes which, at an infinite number of points throughout time, branch out into an infinite number of unique universes, which all obey the same mathematical laws. If the universe is a mathematical structure, and time did have a beginning, then that beginning is just the minimum possible value of whatever measure corresponds to time, where there is a "universe function" of time and a bunch of other variables. Although, it might be that the universe at the present is defined as the result of an infinite number of different variables that are "decided" at each point in time (for each quantum event), or there could be a particular uncountably-infinite-dimensional variable which, when plugged into the universe's function, defines the universe at any given point in time. Either way, the universe would still be a mathematical structure.

TL;DR: The mathematical universe is the only thing that is unconditionally existent, therefore the universe exists only because it is a possible mathematical structure.