Because this was literally the definition of a vaccine until it was changed to include these ones a year or so ago. “Makes you immune” is what the definition used to be. If you are immune you cannot spread.I've been seeing this a lot recently. Those opposed to the vaccine, using the word prevent to make a point rather than reduce.
Why does it have to prevent the chance of catching or spreading? If it reduces the risk significantly, which to my understanding it does, why is that not seen as worthwhile? Many things reduce risk rather than fully prevent which we all go along with in day to day life.
It needs to prevent you catching it and spreading it so that a)the disease goes away and b) so you don’t create an evolutionary bottleneck inadvertently making the disease worse.
Leaky vaccines are dangerous which is why, for good reason, they have been banned for use in livestock for a long time.
And most importantly, if we can all admit that it doesn’t stop the spread, then there’s no reason for any of these measures, it makes it a personal choice, not one for the collective good. If you feel you want to take it, then yeah go for it, if you don’t, then don’t.