The padded chamois is meant to absorb the sweat, underwear would have seams that can chafe on longer riders, and of course absorbs sweat, but most would still end up being absorbed by the chamois. So, wearing underwear does not allow one to necessarily get multiple wears out of before washing, unlike street clothes where one is not sweating. Wear, wash, repeat. If you'd like a suggestion of a budget friendly short on Amazon, I'd be happy to help.