(1) Previously, you had to build your entire system around one of the simulation frameworks (and then not take any dependencies).
(2) It’s way too easy to fool yourself with weak search/input generation, which makes all your tests look green when actually you aren’t testing anything nontrivial.
As you say, Antithesis is trying to solve both of these problems, but they are very challenging.
I don’t know of anybody else who has a reliable way of retrofitting determinism onto arbitrary software. Facebook’s Hermit project tried to do this with a deterministic Linux userspace, but is abandoned. (We actually tried the same thing before we wrote our hypervisor, but found it didn’t work well).
A deterministic computer is a generically useful technology primitive beyond just testing. I’m sure somebody else will create one someday, or we will open-source ours.