Because people have poor taste.
I get you, it's so hard to find good teen and adult bat fanart.
I consent with you anon. Even though it is clearly the best form.
Well, I wouldn't go as far as to say it is the 'best' form. You see, loli Shinobu has a special charisma to her aswell.
If I had to choose one I would go for 10 y/o, bowl cut Shinobu.