They do that for legal protection. Especially mainstream food companies, they don't want to label themselves "vegan" because they want to sell products to a wide market, but they also don't want to get sued by vegetarians or vegans (or by people with nut allergies or milk allergies, that's a thing too). Oreos by Nabisco is a good example of this, they actually state on their website it's for legal reasons, they're totally transparent about it since a lot of vegans call Oreos "accidentally vegan."
There's also the aspect of "99.9% vegan" and that means sometimes it's easier to save money as a vegan if you purchase something on sale that might have refined sugar or unsustainable palm oil. Halloween candy and Top Ramen are examples of this. There's really no way to know if SweeTarts contain bone char, or if Top Ramen sustainably sources its palm oil, but since it's less than 1% of the overall product ingredients, it's not on the same level as purchasing a big vat of unsustainable palm oil, or even a whole bag of refined white sugar. There are vegans who avoid the "99.9%" but most of them are either affluent enough to afford it or have been vegan long enough that it's easier for them. I know as a college student there are times where I buy the plain Rice Ramen and there are definitely other times where the vegetarian Top Ramen soy sauce flavor is more within my budget. 30-40 cents per ramen cake vs. 1 dollar per ramen cake doesn't sound like much but there are weeks where it makes a difference.
PETA actually promotes "99.9%" because it makes veganism more accessible to everyone, whether they're a college student, a working class family, or an individual living in a marginalized community where corporate products are mainly all they have to choose from.
