QF's flight is a classic QF publicity stunt which will generate hundreds of thousands of dollars worth of free media. But that aside, in Australia it's not only "in their own countries" it's "in their own states". QF are tapping into pent up demand, at the moment those in SYD can only fly intra-state (although that looks like changing tomorrow with the border opening up to one other state), without incurring dreaded hotel quarantine charges. And no international travel without an exemption (and at the moment, no way home unless you pay full business class or F, or are very lucky).

In Singapore, people can travel internationally, but more or less travelling to any destination that will actually allow them in, on return they face a $1500 USD quarantine (or $1900 USD for 2) in a hotel room for 14 days, where they can only leave the room once - for COVID tests. Add in $130 USD/pp COVID test. And then be on the hook for all medical costs should they need treatment for COVID on return (unless they are privately insured, which isn't very common). All in all an expensive and risky option. Hence flights like this on SQ may appeal to some, with no need to cross borders. But their marketing machine isn't quite as slick as QF's, so they haven't confirmed such flights yet.

From what I tell these dynamics don't exist in the US, people can actually get on planes on fly somewhere without facing 2 weeks in a hotel room they can't leave on return! So don't see the appeal of UA doing such flights.