Vegans would answer that firstly by saying their conscience requires them to be vegan despite what anyone else may be doing, as it is a matter of choosing not to participate in industries that rely on animal suffering and death.
However, sometimes it can feel like your efforts are a drop in the ocean in terms of ending animal abuse when so much of the world uses and consumes animals, and this can be demoralising. If every person throughout human existence had thought this way though, no positive social change would ever have happened.
The concept of minority influence (source) tells us that individuals and small groups can start to make a difference in the world, and the vegan movement is expanding and becoming more visible all the time. By becoming vegan, you would not just be ‘one person’ choosing not to use animals, you would be joining the growing ranks of hundreds of thousands of people who have already individually come to the same decision.