Who says they will? They just gained market share and more ad dollars.

Political ads are by rough estimation about 1.1% percent of all revenue

(calculated from https://articles2.marketrealist.com/2019/05/why-facebook-con...)

So not a lot, but still it's 1%

"Gained market share" is incidentally not what FB needs right now when they are already under heavy scrutiny for their political ad policies.