Probably because most people know their gross income off the top of their head (their hourly rate or salary), compared to having to mentally calculate deductions from taxes, benefits, etc.
Also the gross number is bigger, and when comparing to other people it feels nicer to say the bigger number. Same reason people round down on their weight when asked and round up when asked about their height.
As far as businesses/banks asking, probably because it’s a quick answer without having to actually calculate net since each person’s net pay is pretty unique based on many factors.
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