libp2p does have QUIC, but it is one of many possible transports.

So libp2p builds many things on top of the underlying transport where we use QUIC directly and use existing mechanisms such as TLS ALPNs for protocol negotiation.

We also use the stream multiplexing that is built into QUIC instead of putting a stream multiplexer on top of QUIC.

You can think about it like this: libp2p abstracts transports as streams, and then puts many required features on top (protocol negotiation, stream multiplexing)

Iroh uses QUIC and abstracts transports below QUIC. We can work with any unreliable datagram transport that has (or can be hacked to have) a minimum MTU of 1200 bytes (needed to be QUIC compliant).