The issue is it looks like the Delta variant is able to infect people who are vaccinated. The symptoms are not severe but it is able to spread in vaccinated people too, so it could mutate in that segment of the population too.
And the rest of world is nowhere close to vaccinated enough to stop the spread.
I look forward to the future research that explains how this virus manages to spread so easily and why it affects some people so severely and others not.
I understood that the early numbers indicate that the vaccines do protect against the delta variant - though not to the same level (probability to get sick) as the earlier variants. The earlier variants are also (!) able to infect vaccinated people, just only with a very low probability. Nothing is absolute here, all are probabilities.
Viruses mutate so much because there are less protections in RNA copying errors (as compared to copying of genetic material in mammals). The covid virus in fact mutates slowly for an RNA virus, presumably be it has some way of qualify control (like mammals, but not nearly as good).