Because antivaxxers don't want to have a nuanced discussion of pro and cons of any vaccine. This iteration is "mRNA is new so it's bad". But it's always been the same and ends with "vaccine is bad" for them. It's not possible to bring a reasonable discussion to someone who will happily throw out reason to accept their own preconceived conclusions.
> In my own case, I had significant cardiovascular issues at a relatively young age, 1 week after I got my booster.
Even before the vaccine, it was clear that covid triggers cardio events in some people, even young. Even three years in, it's not clear why that is. So it's plausible that a vaccine's spike protine could trigger an event, above the background level. The question then is, does the vaccine trigger cardio events at a higher or lower rate than getting covid?