Why?
For Instance in 3D Game Kit you have ready to use meshes and scripts which you need to tailor to your needs, e.g. replace rock meshes with ones you like, paint a new level and play.
While starting from scratch looks like reinventing the wheel.
My question actually is as follows: do those Unity kits evolve in time (I mean latest is how it should be done in 2021 and 3D kit is old obsolete 2018 toy) or I just take which I like more and play around.
As we know, old good assets and projects get compile errors when opened in modern Unity versions…
3D Game Kit is a fine place to start. But be prepared that it has some glitches and a clunky structure overall. You will probably have to do a lot of tuning and code rewriting if you want to make a game that is not just Explorer with replaced models.
I don’t start from scratch but I don’t start with the kit as a base for my project either. Instead I determine the assets in the kit that would benefit my project and extract them and their dependencies from the kit. That way I don’t have to live with all of the design decisions introduced by the kit.
I haven’t tried “microgame” but as for FPS Kit, it’s the same as for 3D Game Kit. You can use it as a base for an FPS game just fine but still have to code. Making games without coding is a myth.