Because discussing veganism is a distraction from the real issue.
In developed countries like the USA the greenhouse gas contribution from all agriculture is about about 11% (the EU is similar). Livestock is a significant part of that (I seem to remember a figure about half but can't find a reliable source right now). It is mostly because the developed world has much higher emissions in general so the contribution from agriculture is smaller but also because of more efficient meat production.
Discussing veganism at this stage is distracting from the real issue, the other 90-95% of emissions. If we don't fix that, it doesn't matter what we eat. If livestock some day accounts for 20-30% of our emissions, then we can discuss veganism but there is a chance that will never happen. Just like in all other sectors, there is ongoing discussion and research on how to limit the emission from meat production. It is too early to tell if veganism is the only answer, or if eating chicken is perfectly fine or if beef can be produced with limited emissions by using new breeds and feeds. But most importantly, it is not an urgent question and we have time to figure out an answer. It is the other 90-95% of emissions that we have to fix now.
By the way, TFA is misleading with the " 16.5% and 28% of all greenhouse gas pollution" claim. If you read the actual linked article, the 28% figure isn't found until chapter 3 and it is NOT an estimate of the current "greenhouse gas pollution". The figure is an estimate of how much a total switch to plant based foods could reduce the carbon budget IF the land used for meat production is NOT used for anything else. Quote:
"the land no longer required for food production could remove 8.1 billion metric tons of CO2 from the atmosphere each year over 100 years as natural vegetation re-establishes and soil carbon re-accumulates"
So, if you use the land for growing crops or building golf courses, the 28% figure does not apply