Pole precession and motion of the perihelion,why couldnt the earth spin and orbit at the same angle for ever?

Answer 1

The earth's spin is like a spinning top that is tipped over 23º. Thus the axis precesses in a small circle as it goes around. If the planet's spin was straight up and down (0º) to its orbit then it would spin at the same angle forever.

Answer 2

Since the Earth is spherical instead of being Round. Coupled along with its axial spin. It behavest like a gyroscope.

The pull on the earth from the sun and moon, is not uniform, because of the bulge at the earth's equator.
The interference in the earths axial rotational momentum caused by the sun and moon, acting gravitationaly upon the uneven mass of the earth
Creates the cyclic motion of pole precession ,
Conservation of Angular momentom is maintained by the pole precession. .

The earth can not remain at a uniform angle with respect to the perihelion, because it's axial spin , distributes it's mass more equatorially at the expense of the poles.., once it becomes spherical
It begins to wobble.

Answer 3

If there were no other forces, it would, more of less. But there are other forces: other planets, acentric orbit about the sun, the earth's changing mass (slight but measureable), the other forces in the cosmos.