That's called contextual targeting:
"
If iOS 14 users opt out, they will still be shown ads, but they’ll be based on other methods like contextual targeting rather than based on their IDFA.
"
https://clearcode.cc/blog/apple-idfa/#apple-skadnetwork
The issue is not effectiveness as much as measurement. Advertisers value the insight obtained by tracking user response to ads. That is something they are willing to pay premium for. Without the ability to do measurements, Facebook does not provide as much value.