VST3 came out 12 years ago. Why do so many developers still insist on VST2?

Of course it all depends on what you have in your plugin collection. The reality these days is that most developers offer VST3 versions including: Acon Digital, Sonnox, SSL, Waves, Voxengo, IK Multimedia, Blue Cat, Melda Production, Klanghelm, Izotope, Softube, Nugen, Antares, Fabfilter and many others. A notable and surprising exception is Native Instruments who do not seem to want to update at all (in my opinion a backward decision).