I understand the consideration that this Universe is all that exists. But isn't that... weird? That there's no 'why' to this particular existence? We could be much more complicated if you consider all possible things that could exist. Is it simply senseless to ask 'why this'?

This view is argued here by Sean Carroll: https://arxiv.org/abs/1802.02231

Personally, I'm not convinced, although I see the merits of the argument.

Of course as you noted, if you simply assume a parent system you get into an infinite that may or may not be reasonable (the parent system must have a parent, and so on, an infinite stack of parents; this infinite stack of parents also lies in another infinite stack, and so on). I think the only possible alternative to brute fact is to just assume everything exists, to different extents. Then after a lot of handwaving you can sort of explain why the universe is kinda simple.

https://en.wikipedia.org/wiki/Mathematical_universe_hypothes...

I haven't picked a side, but I have to say I'm more weirded out by the proposition that this is everything that exists than the proposition that essentially everything exists.