Same reason people wear polo shirts when not playing polo, rugby shirts when not playing rugby, trainers when not running, and denim jeans when not wrangling or mining. Because as a form of working dress they're comfortable and convenient.
They are still a relatively expensive piece of protective clothing, though I suspect somewhat more robust than they used to be.
People have already tried and failed to impose a 'if you're not flying then you are to wear blues' rule at various RAF stations and it's failed. Not only is it a pointless rule only valued by jobsworths and folks who appear to be envious of having something comfy to wear (I'm sure the same rule should apply to expensive protective MTP combat wear as well no?), but also would be another one of the small annoyances that is driving people away from staying in the military.
