Because most people who go 100% vegetarian break down and go back to eating meat anyway. Better be 98% vegetarian for decades than 100% vegetarian for a year.
Maybe in the past things were different (although many studies into this tend to have ulterior motives), but I would say that Veganism has gone from an obscure lifestyle/diet to the acceptable mainstream very rapidly - e.g. U.S's population has gone from 1% vegan to around 6% in just the last couple of years (https://www.forbes.com/sites/janetforgrieve/2018/11/02/pictu...).
Even KFC are creating a vegan option.. sometimes change happens quick. I see it becoming more and more normalised.
I think people should really just aim for 50% vegetarian, I really think this would help and most people could manage. For example, eat meat 3-4 days a week and the rest vegetarian. Imagine if everyone did this?
Yeah it's not perfect, but it's a start and it's more realistic.
Here's my story and I've heard similar off others:
I went vegetarian for 2 years (leaning to vegan) and found it hard to keep in shape and get enough protein, especially if you don't have time to cook and prepare meals. The only time I felt good was when I worked remotely and really had time to invest in cooking a lot of legumes and preparing meals with a lot of spices and variety to keep tings interesting.
Eventually I moved overseas and worked full-time again and felt depressed because I was missing out on a lot of nice foods and was surviving off only Tofu (readily available protein source), the lack of variety got me down.
I felt hungry quite often and often ended up supplementing with things like whey protein (probably has high Co2 footprint) anyway.
Eventually I slipped into some kind of extreme depression and was quite unwell mentally, I pondered a lot for why this might be. I'm not sure if my body was deprived of something and then I thought back to when it all started and it seemed like a correlation with my diet changing. I hate saying (because I think the ethics of vegetarianism/veganism is superior) but my mental state improved quickly once I started consuming more animal products higher in protein with more variety of amino acids (whey protein, cheese and milk combined with lots of nuts and legumes).
Eventually I went back to eating some chicken and fish and took some whey supplements and things continued improved for me and now I'm really quite content and working out / lifting a lot and feeling good physically and mentall. I eventually I just realized that I need to eat. I care a lot about climate change so I felt insane guilt for a while but it went away eventually.
Like someone else said, maybe it's enough not to treat meat like a commodity, more like something we're lucky to have an eat it as necessary and sparingly.
I think it's unfortunate and unhealthy to have to give up food and feel guilty for eating because people choose to drive their cars, have too many kids, drive to work, fly unnecessarily and burn fossil fuels to generate power. I'd prefer to see those issues addressed before going hungry and having health issues.
I know people live vegetarian fine, maybe it's easier if you have a low body mass etc; However in the end my contribution has been to move very close to my office and ride to work every day and I eat vegetarian for breakfast and most evening meals, consume less stuff I don't need, repair things and only fly once or twice a year (maybe it's too much).
Even KFC are creating a vegan option.. sometimes change happens quick. I see it becoming more and more normalised.