@Robin4 Viruses mutate when they replicate i.e. make copies of themselves, since the DNA copying process doesn't always happen exactly 100% the same each time. The more the virus replicates, the greater the chance of a mistake in copying, i.e. mutation, i.e. variant, occurring.
In other words, the greater the number of people that get sick, the more chances there are of mutations and thus variants.
This is why influenza is different every year.
If everybody got vaccinated, obeyed mask mandates, kept social distancing and took all the other recommended precautions, we'd reduce the number of potential mutations because fewer people would be getting sick.
Covid is not going to go away. It is just going to keep changing all the time, as long as people are still catching it, and passing it along.