But nothing is real and everything is meaningless

No, it doesn't really mean anything. We are only specs of dust in the infinite cosmos.

But I am a speck and I experience this. I have this subjective experience of existence. I know what it feels like to be me. I know what it feels like to experience joy and I know what it feels like to experience pain, and suffering. I know what it looks like to experience joy and pain.

There is this thing inside me called empathy. I can see others, outside of me, that are similar to me, and I can recognize joy and suffering in them.

There is this other thing inside me called compassion. This gives me a desire to help these others to experience joy and to prevent their suffering, in what way I can.

And so I will fight for these animals, who clearly experience suffering when you take a closer look, to help them to avoid the pain and death of captivity, brutality, sickness, and slaughter. The expense to myself is one of convenience; there is no cost to my vitality, health, or joy. The reward to myself is alignment of values and action; the reward to the animals is their life.