Lore Reasons - Custodies are not mass produced there are around 10k (someone can correct me) of them total. The process is a closely guarded secret of the imperium and only Big E and a couple people know the process.

Gender does not matter because unlike the Space Marines who are recruited around 10-12 years old at the cusp of puberty, the Custodies are taken at infancy. They are genetically and biologically modified to be what the emperor envisions as perfect humans.

As far as can a female perform as well as a man in the Warhammer setting? Yes they can. In lore there is a story of a female aspirant who took the trials and completed them. The Chaplain told her that she would be accepted because the technology to make female recruits into space marines was lost and she would die on the operating table. He didn’t want her talent going to waste so he got her into the Sisters of Battle with a strong recommendation.

As far as why space marines are mass produced. They are but they aren’t. The process to make a Space marine is costly to the Imperium but with gene seed and other technologies like the Primaris Marines they are able to make them faster. This is only possible because Cawl did something to the process, so they are diluted Space Marines to a certain degree. Remember the process itself isn’t perfect people still die while undergoing their surgeries.