There are a few reasons. First, cows are forcibly impregnated and their babies taken from them to slaughter. Then after a few years of this cycle the mothers are also slaughtered. The same with hens who no longer produce eggs. This is why vegans say things like “The egg and dairy industry IS the meat industry.”
Second, animals are bred to be unhealthy machines. Turkeys so fat they can’t naturally mate, hens that produce 5x as many eggs as they naturally would, rapidly depleting all the hen’s resources, sheep that produce way more wool than they would, cows with udders so big they can barely stand, etc. We see the same with dogs bred to be attractive to customers but unable to breathe properly (pugs). Christian Veganism takes all these things to be irresponsible stewardship over the earth and its inhabitants.
Thirdly, even if we did not breed the animals this way or treat them badly for selfish gain as we presently do, it comes down to faith and obedience to God’s original design and instructions for us. He said in Genesis 1:29 our food should be fruit and vegetation and then in verse 31 that he saw everything and it was “very good”.
Ever since the fall God has called us to repent and turn back to his ways. Nowhere else does God call creation “very good,” and he promises in Isaiah, Hosea, Genesis, and elsewhere of a future kingdom of peace for people and animals alike. He even makes covenant promises about it with the animals. So even if there were a way to circumvent all the animal cruelty and exploitation, it would still come down to faith in God’s original design and in his promises of the restoration of that original design.