Why can’t they just patch up the kernel to persist PRNG state there and require that the boot loader recovers it upon reboot? The kernel would then have high quality random data available as soon as it gets launched.
Imagine a virtual machine snapshot that you start multiple times. The separate runs would have the exact same persisted state.
When you are dealing with an attacker which can feed your PRNG non stop with their own entropy you have other issues to deal with.
Either you think there's plenty of usable entropy soon enough after power on that values can't be predicted, or you think there isn't and we need to keep a file to initialise from
> an attacker which can feed your PRNG non stop with their own entropy
Is not needed if you know the initial state from the file (which was present in the image file an attacker got their hands on, for example): knowing a sufficient percentage of outputs (such as the tcp seq/ack number (I forget which one the server generates) when you send a decent rate of syn packets) lets you verify which entropy addition would have lead to that output being generated. That's a handful of bits to brute force at each step unless the entropy additions are processed in prohibitively large batches (say, ≥80 bits at a time) which means you start completely predictable until that entropy is accrued (which may or may not be an issue depending on circumstances)
There are many new attacks that could surface if import/export were possible; off the top of my head: quiet preloading of an attacker-selected state while the machine is off.
That attacker with physical access can chose from multiple other attacks, so why bother making this one more secure. They can sniff keyboard strokes, install malware into the UEFI, etc. pp.
Just preload the OS then.