Because they're in the middle of Phase 3 of the trials which do have a Phase 4.
Pfizer's clinical trial; https://clinicaltrials.gov/ct2/show/study/NCT04368728
Moderna's COVID-19 Clinical Trial Phase 3 at clinicaltrials.gov; https://clinicaltrials.gov/ct2/show/study/NCT04470427
Pfizer's Primary Completion Date; Nov 2021 -- Study Completion Date; May 2023
Moderna's Phase 3 Completion Date; Oct 2022.
Phase 3 trials include healthy people 18-55/65 who have not had covid and pregnant women are excluded as well. That's for the trials though. In the public, there are no exclusions to who's getting the shot.
thanks