1) why not common ancestorS as in PLURAL?
Because that assumes a completely unneeded other explaination. It's like saying well, I'm sure the car drove from Leeds to here, but I think it flew from Glasgow to Leeds.
2) why not common parts as evidence of a common designer
Because that describes nothing like what we observe. The designed objects we can observe
do not look like the natural organisms we observe, and they certainly don't form heirachies in the same way. In particular, a designed object will tend not to use multiple different solutions to a problem (unless it has multiple designers), yet every cell in your body uses two different forms of DNA encoding.
3) Why not just Stop this conclusion: 'in our investigation of the fossil we found animals, plants, etc of having ALMOST similar parts?'
Because it's not true; there are enough similarities between
all forms of life to conclude that they all share a common ancestor.
And because it's a deeply unsatisfying conclusion. Especially when the extrapolation is so obvious.