Because the number of people who can actually validate the security of the open source options is vanishingly small and assessments like this provide sufficient evidence that WhatsApp's claims are not bunk.
Actually, no that's not sufficient evidence. That's only sufficient evidence for those interactions between the auditors and whatsapp's servers, it doesn't say anything about your interaction with whatsapp's servers. And that's the heart of the problem: using code shipped to a browser for e2e encryption is a flawed model, it would be trivial to target you with a version that has broken e2e encryption but subtle enough that you would never notice.
The people who can judge if Facebook are to be trusted should be in the millions by now though...