Why do planets have a tilted axis?

Answer 1

Tilted, as compared to what?

The idea that there is a "correct" orientation for a planet is false. Each planet formed independently of the others, both in the original coalescence and in the effect of the myriad impact events that each planet surely experienced. Seven of the planets have an axial tilt (compared to the plane of the ecliptic, which is the plane of the Earth's orbit) that isn't especially different from Earth's. Only Uranus is way out there. Because all of the planets formed from the same planetary disk which surely had its own inherent rotation, all of the planets orbit in pretty close to the same plane. Of course, over the past 4.5 billion years, chances are that any object that was in a wildly different plane would probably have collided with one of the planets, or been ejected from the solar system completely, or perturbed into the Sun.

Answer 2

Many things about the cosmos don't have a "why" about them; they just ARE.

Most of the planets have an axial tilt to them. We can GUESS why - that a titanic collision early in the planet's history knocked it that way, or that that's how the planetesimals that came together to FORM the planet were spinning.

But we may never KNOW why.