Because myocarditis is not myocarditis. The cases observed after vaccines were mostly mild. The myocarditis after vaccines does not necessarily have the same severity as myocarditis in general, and from observations so far it looks to be milder:
> In COVID-19 mRNA-vaccine-associated myocarditis, >90% of patients will functionally completely recover, usually after a chest pain syndrome (see Supplementary information). To date, only eight deaths owing to COVID-19 mRNA-vaccine-associated myocarditis have been reported (>99% survival) (see Supplementary information).
> In COVID-19 mRNA-vaccine-associated myocarditis, >90% of patients will functionally completely recover, usually after a chest pain syndrome (see Supplementary information). To date, only eight deaths owing to COVID-19 mRNA-vaccine-associated myocarditis have been reported (>99% survival) (see Supplementary information).
https://www.nature.com/articles/s41569-021-00662-w
> Most patients with myocarditis or pericarditis who received care responded well to medicine and rest and felt better quickly.
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/safety/my...