Chrome checks for Signed Certificate Timestamps (SCTs), which are proof from a CT log that it received a certificate at a specific moment in time. There are several ways that SCTs could get to the browser, but the most common by far is that the CA obtains them by issuing a pre-certificate (currently this is a “real” x509 certificate that is deliberately “poisoned” so that it won’t work) and then includes the SCTs for that inside the certificate it issues for actual use. That way nobody needs to upgrade to newer server software, just the certs they obtain are a bit larger.
Although Let’s Encrypt certificates are already voluntarily logged (over 100 000 per week) with several CT logs, they do not provide SCTs in the certificates they issue. Given that LE has lots of other stuff to focus on, I would guess that they prefer to wait until 6962bis (an as-yet unnumbered successor to RFC 6962 which will standardise CT permanently, RFC 6962 is an “experiment” not a finished standard) is completed.
Voluntary logging, with no client enforcement is still valuable because it lets everybody see if LE make any mistakes. Obviously if they were actually Evil they’d hide their evil from the logs. But if they’re just screwing up (e.g. when they issued despite CAA mis-matches last year for a little while) with voluntary logging we can see it. Other CAs seem to be at least somewhat coming around on voluntary logging.