> Why not penalize by percentage?

The ACCC usually does do it this way.

Some recent examples: https://www.accc.gov.au/media-release/air-nz-penalised-15-mi... - Air NZ penalised $15m https://www.accc.gov.au/media-release/iphone-and-ipad-misrep... - Apple fined $9m https://www.accc.gov.au/media-release/domain-name-corp-and-d... - Domain Name Corp/Agency fined $1.95m

You'll see that most of these amounts are pretty close to the amount the companies made from the activities and scale with the size of their operations