It says so right here in the Laws of Physics:
147(a) No pine tree shall grow to be be perfectly vertical. If it appears so, it is an optical illusion which will be spoiled by viewing from an angle of 30 degrees.
147(b) No Christmas tree holder will tilt in the direction necessary to make the tree perfectly vertical.
147(c) It is impossible to cut the base of a pine tree to compensate for the lack of perfect verticality.
I have proof, if you wish to drop by to see it.