Because not everyone agrees that it's terrible for Earth. And even some of those that do may not consider it so terrible for Earth that it's not worth the tastiness.
You're wasting electricity running a computer right now, when we know that electricity generation is terrible for Earth. Why are you doing that?
Humans can be weird about these facts or simply indifferent to the known effect that raising these animals for meat has on the environment. Additionally, I think the antagonistic message of a few vocal vegans triggered a powerful foolishness in the heads of certain people who are prone to acting hedonistically upon being told not to do something. A combination of apathy, chasing profits, taste for beef, and spite which fuels the industrialized beef production business. Another issue is that most of us simply won't be around to experience the consequences of the unchecked corporations responsible for this willful harm the meat industry is causing Earth's climate and surrounding environment. I believe in moderation, eating as little of all the meats as possible (those industries have a big impact on the environment). As an American, I see a weird pride that certain people have about eating as much meat as possible; loudly shunning and making fun of those who have either a mostly plant-based, vegetarian, or vegan diet. It's such a selfish outlook that happens in societies that focus on the individual over the many.
Another issue is that most of us simply won’t be around to experience the consequences
I think most people middle-aged or younger will experience the consequences (in fact, we already are with the increased frequency of severe weather events) it's just that those consequences will get worse over time.
The youngest of people certainly will be I could have worded this thought better, but I didn't. Severe weather is certainly a consequence as well as increased extremes in temperature which are currently happening. Everyone already feels the impact of irresponsible environmental decisions made by the oil industry and industrial agriculture/animal husbandry. Millennials, Gen X and Gen Z will be around to experience the worsening of conditions on Earth. I do genuinely believe that people don't consider the fact that they aren't going to experience the climate outcomes based on irresponsible decisions. However, based on the current growing political instability of the USA; I wonder if people are beginning to feel a desire to indulge as they don't know if they'll come out unscathed from the blowup which is bound to happen at some point. A bad outlook to have in a way as that will only magnify future issues, however, humans aren't always rational! 🤪