Why do we use CLTV instead of CSV in the HTLC output of the commitment transaction? In lightning network commitment transactions, the HTLC offered/received outputs generally pay the offering node after a timeout which is scripted using CLTV. In other parts of the commitment transactions (such a to_self_delay) we use CSV to ensure it the output is unspendable until a certain time has passed. Why was CLTV chosen as the design choice for HTLC outputs?
As you have said yourself, CSV is relative to when the transaction appears in the blockchain, while CLTV is absolute. They just have different meanings, which are useful for different things:

The RSMC timeout is meant to only start when someone pushes a commitment tx to the blockchain. So it needs to be relative.

The HTLC timeout, on the other hand, is meant to actually make an HTLC temporary, even if in most cases if will never hit the blockchain. So it needs to be absolute.