Volcanoes do change the climate - especially large eruptions. But they are part of the natural carbon cycle which operates over million-year timescales. Their CO2 doesn’t build up monotonically - it is also absorbed. So it would be fallacious to draw an equivalency between volcanic emissions (small emission times large duration) and anthropogenic emissions (large emission times small duration). The rate matters.

Anthropogenic CO2 will also be reabsorbed over millions of years. But the rate of emission is so high that our challenge is to survive the next few hundred years.