Have you heard of delegation?
Delegation and deputising happens in most jobs, especially for those at the top end of a business. I suspect this is very similar. The Queen can't really retire - retirement isn't part of the role. She'd have to abdicate which isn't going to happen and abdication isn't as straight forward as her stopping and Charles taking her place - there's more legalities to it.
Instead this is the work around - she doesn't what she cans and senior members of the RF stand in for her. This happens in lots of family businesses too, if you want a 'work place' equivalence.
There's also a huge difference between sitting in her own car in her own grounds looking at some animals, than travelling to another location, standing in front of others and delivering a speech which takes much longer and far more effort.