So I’ll give you my take, and it’s by no means exhaustive, though maybe a bit lengthy, so TLDR; veganism is on a do or don’t spectrum; piracy is (to many) more nuanced. Here goes:
For one, you can download software, but you can’t download, say, a cow. There’s definitely a distinction to be made between tangibles and intangibles. In addition to that, they’re different types of commodities; food is a necessity, software is a luxury (IMHO).
In my experience, the reason consumption of meat (or not) is often such a contentious subject, has to do with how vegans position themselves on a moral high ground, based on their belief that no living creature deserves to be killed in order to satisfy our human need for food. If you consume, you implicitly kill, and so you have no escape from the morally condemnable bad action/inaction.
Since most of us aren’t farmers or involved in husbandry, we need those people (by extension the industry) to provide slaughtering, refining, etc. as services in order to enjoy the final product. While you could in theory keep your own animals, laws prevent you to do so practically, due to agricultural regulations and sanitation requirements (may vary depending on where you live, I dunno).
When it comes to software, there are basically two camps (AFAIK): those who believe tech companies/devs charge reasonable amounts for their products or recognize and advocate buying as a way of supporting good products; and those who believe tech products are overpriced, and/or that the companies/devs engage in anti consumer practices in one way or another and consequently pirate.
Pirates necessarily go against the industry, and due to the nature of many tech companies actually employing shady business tactics, the grounds for arguments are a lot greyer, and so there’s a lot more back and forth; trying to determine if something is acceptable and under which circumstances. Many times people conclude that not supporting the industry is the better move, often considering not a single producer/Dev but the market/industry as a whole (often because of megacorps), and sometimes that giving support is the better move (you’ll see this sentiment a lot more related to indie devs, open source and federated stuff).
For many people, rather than not consuming a product, consuming it in an unauthorized way is a better way to flip the finger, and so it sometimes comes down to a tooth for a tooth response. Many people don’t have the luxury of buying products they don’t know are right for them or have the featurs they’ll need, and turn to piracy as a form of demo/trial, buying only if the product meets their needs/standards.
In the end, most people I’ve come across (who are vegetarian or vegan) view their choice as a moral “should”, because they consider animals equal to humans in terms of rights to life. It’s a do or don’t scenario, and if you do, you are by extension morally worse because you feed the industry (necessarily). People tend to have a more nuanced view on piracy because you’re not directly affecting someone’s right to life, and sometimes piracy can be construed as a form of protest. As such, you could argue that piracy, although not technically good, is in some cases bad, and in some cases worse.
This is the post source, in case anyone’s doubtful https://news.cornell.edu/media-relations/tip-sheets/cornell-expert-says-trumps-frequent-phonemic-paraphasia-are-signs-early