In the beginning they partnered with an existing CA so that they could issue certificates that where chained to roots already trusted by the major browsers.
“Getting a new root trusted and propagated broadly can take 3-6 years. In order to start issuing widely trusted certificates as soon as possible, we partnered with another CA, IdenTrust, which has a number of existing trusted roots. As part of that partnership, an IdenTrust root ‘vouches for’ the certificates that we issue, thus making our certificates trusted.”
“Getting a new root trusted and propagated broadly can take 3-6 years. In order to start issuing widely trusted certificates as soon as possible, we partnered with another CA, IdenTrust, which has a number of existing trusted roots. As part of that partnership, an IdenTrust root ‘vouches for’ the certificates that we issue, thus making our certificates trusted.”
https://letsencrypt.org/2015/10/19/lets-encrypt-is-trusted/
https://letsencrypt.org/2016/08/05/le-root-to-be-trusted-by-...
https://letsencrypt.org/2023/07/10/cross-sign-expiration/