Masking is not always conscious, in fact it's largely unconscious. So many autistic people will go through their day around neurotypical people and feel burnt out by lunch and have no idea why. They don't necessarily realize they're burning tons of mental effort just talking to people or dealing with stimuli.
Autistic people learn to mask just to get by day to day. It's not like they got issued a "How to be Autistic: Masking for Success" guide book when they were born.