Why not, out of curiosity? We don't make this distinction for low-income folks who get parking tickets. If you don't make them truly punitive why comply? It just gets factored into the cost of doing business.
But fines and tickets being pegged to income is a good idea. Fixed amounts for fines are really expensive if you don’t make much money. Similarly, they aren’t much of a deterrent if they are a small amount of your income.
Facebook has a distinctly high profit ratio (2020 Facebook revenue was $85 billion, net income $29 billion), so 4% of revenue means a lot less to Facebook than other companies that GDPR restrains. 4% of their revenue is $3.4 billion which is 11% of their net income.
And the potential negative impact of Facebook's negligence in data handling is distinctly greater than other companies, Facebook collects far more data than most other companies and has access to far more personal data. They have somehow managed to convince a huge portion of the world's population to share a significant tap of data into their lives.