Sadly, since the use of asbestos was stopped when making brake pads, the make up has changed, where the discs are softer than ever in order to get decent performance.
It's not uncommon for brake discs to outlast brake pads.
Since the metal is 'softer' than previously, it's more prone to rust. This is especially bad on the inside edge of the disc, where the water can sit.
Cars sitting unused outside suffer...as do those washed and put away without drying the brakes off first.
This can be minimised by driving the car round the block with plenty of braking, before laying the car up for a lengthy period, to ensure the insides of the brakes are dry.
I suffered this exact issue with a previous car...and since drying the brakes before periods of inactivity, I've avoided the problem.
Sadly, rusting discs is something we just have to live with.