Scott had already stated that anyone entering the chamber would be dead in under a minute.
"It isna possible, sir!" Mr. Scott cried. "The radiation level is far too high; i' ha' already burned out the electronics o' the repair robot, and if ye went in in a suit 'twould freeze for the same reason! A person unprotected wouldna last a minute!"
Star Trek: The Wrath of Khan - Official novelisation
Spock enters the chamber and Bones and Scotty both yell at him to come back out again but only for a few seconds (while it's still possible that he might make it out alive). After that they simply watch in "helpless horror" as he completes the modifications and fixes the engine.
HIS FACE: His face, using all the self discipline he is capable of to control the pain, continuing to work --
WITHOUT: Bones and Scotty react in helpless horror.
Wrath of Khan - Original Screenplay
Since Spock has already taken a lethal dose, beaming his body from the chamber would simply expose the sickbay personnel to radiation for no good purpose (since they can't possibly heal him) and there's a job that needs to be done. Once he's finished, It's clearly safer all round to just wait a few more minutes for the decontamination cycle to be completed and then drag Spock out in a lead-lined bodybag.