It's the commanders prerogative or right to allow or not to allow an F/O to fly the machine but it's generally accepted sector 4 sector unless as has been mentioned here Capt only at certain dromes & or during certain WX conditions.
The so called 'shortage' is for 'suitable' pilots, any monkey can fly a plane pretty much but Airlines these days want the 'right' person for the job not necessarily the best pilot for the job, hence a 'shortage' in some eyes.
Wmk2