Well because what it does in your body is very logical and basically the same as with traditional vaccines: your body produces antibodies, stores info in memory B cells and destroys the mRNA. The mRNA is then gone within hours so it can’t cause any effect in the future. The only effects that it will cause will be few hours after the shot. After that it‘s just gone.
Antibodies and memory B cells are created upon detecting the newly created (and later destroyed) spike proteins just the same way as in a traditional vaccine, so there‘s no added risk there. We know exactly how the antibodies and memory cells behave since they are created in the same fashion as always. Just a reaction to new unknown spike proteins.