I can. There are two reasons.
First, if you are unvaccinated, you are much more likely to catch it, just like with the flu. Unlike the flu, though, you can catch and carry Covid without knowing that you have it, meaning you won't stay home sick, greatly increasing the chance that you will run in to your neighbor at the store or the bank or work or whatever. Even if your neighbor is fully vaccinated, that doesn't make him invulnerable. Even if he only has a 1% chance of catching it from you, the more people like you he passes by, the more chances he has of rolling those boxcars and catching a breakthrough case.
Second, viruses mutate. That's how we got the Delta variant. The more people who have it, the greater the chance that we get an Omega Variant (or whatever) that is even nastier and even more vaccine-resistant, and then we go through all of this again, maybe losing millions this time.
There you go.