Yeah, part of the whole concept of a PKI is that you generally don’t need to tell people about root certificates.
As is often said, if they trust the root certificate, then they already trust it, while if they don’t trust it, being told about it won’t convince them to trust it.
So for example, for the DST Root and ISRG Root, Let’s Encrypt doesn’t tell people about those because users must already have one of them installed in advance as a trusted root in order to accept Let’s Encrypt certificates. If they don’t, then they should reject Let’s Encrypt certificates because they have no basis for trusting them (unless they performed their own investigation of Let’s Encrypt’s certificate-issuing practices, in which case they could choose to directly trust the Let’s Encrypt intermediate cert, which is served up by the Let’s Encrypt ACME endpoint).
The certificate chain that Let’s Encrypt provides users is essentially “things that clients may not know about yet but that, if they see, they’ll most likely accept”. Including any root certificate wouldn’t align with this because either the client already knows about a root, or the client shouldn’t accept that root.
This is also the case in a private PKI! The clients that are going to consume the certificates should already have had the appropriate roots or intermediates distributed to them via some other channel. Boulder doesn’t have a way to facilitate that channel from software in part because it represents an organizational problem that probably can’t be solved by software (although perhaps in cases of centralized administration of desktops, there might be software tools that would be relevant and someone could create a tool to integrate with Boulder somehow in order to push out an organizational root).
In the latter case, the root still isn’t needed at issuance time for the individual end-entity certificates, but rather when the Boulder instance is created and configured (whatever mechanism will be used to distribute the root should then begin to be used in conjunction with the new CA).