If nothing else Windows is a much bigger target and as the default OS choice it probably caters for more users who could be characterised as "easy marks" - why would the malware writers bother working harder when there is so much low hanging fruit?
That said we use a Mac as the family desktop and run AV on it, partly because there's no real reason not to (overhead isn't a concern) and partly because of the users (young kids with clicky fingers and a missus prone to the occasional lapse in judgement). Probably more belts and braces than absolutely necessary but as I'm the one who would be fixing the mess it works for me.
FWIW I'm using Avira, although reading recent reviews it's slipped so I might need to review my choice.