Because the RN didn't have a true all-aspect front hemisphere radar missile such as Sparrow or Skyflash - which they would have had if the 'real' carriers with the naval F4 had been available.
SHAR wasn't a true alternative, the fact that it did so well in the Malvinas was down to some very good short range combat kills with the new AIM 9L. Was it ever fired radar-slaved in the front hemisphere? Or just in beam and stern shots. 2 risks there - either the bad guys get very close to the Task Force or you have a very short time on a long range CAP in a single engined jet over the balmy South Atlantic in the austral winter. Nice...
Whereas the F4, not perhaps as agile as the SHAR, had a vastly better MCS for fleet defence. Plus it could carry 8 missiles...and it had 2 crew...and 2 engines!