The idea is that cows, and all the infrastructure around raising them, are a huge source of greenhouse gasses, while just cultivating cow meat eliminates everything not directly related to producing a hamburger. More recent studies [1] cast doubt on that and even suggest as you do that cell-cultured meat may be more environmentally damaging, not less.
One of the main factors: generating the growth media for the cells is by no means carbon-free, and some of the ingredients are very very carbon intensive to produce.
Carbon intensive. Most of them are agriculturally sourced, and fossil fuels/feedstocks are not likely to be widely replaced in agriculture for several decades, at least.
[1] https://www.biorxiv.org/content/10.1101/2023.04.21.537778v1....