If they were only bringing the katra to Seleya, then a body was not needed.
The novelizations explain it more fully. The original ritual Sarek wanted for Spock was where the katra, in temporary storage in another's mind, would be temporarily reunited with the deceased body before its passage to the eternal Hall of Ancient Thought. IIRC, normally this ceremony only happens for certain Vulcans anyway. Not everyone's mind is pre-selected for this honour. Unknown to Sarek, Spock had seemingly chosen not to participate
because his will requested burial in space.
Of course, the novelization of ST II was written long before anyone knew what events would unfold in ST III. Thus we learn that Saavik had secretly reprogrammed her mentor's torpedo/coffin so it would definitely softland on Genesis, not burn up on entry to its new atmosphere.
Due to last minute editing, the scene in ST III where Spock's intact tube is located on Genesis is switched to later in the film, so seemingly Sarek has arrived on Earth to admonish Kirk for not agreeing to Spock's wishes - not realising that Kirk had fulfilled Spock's request for burial in space. Sarek assumes Spock melded with Kirk, not realising that the radiation chamber had separated them. Kirk realises that McCoy's strange behaviour was due to Spock melding with him
and they go to check the records.