Matter is subject to "weak bonds". Single crystals can have strong bonds. But if you have a large object (like a planet) it is unlikely to be made of a single crystal, repeated over and over again.
Especially when you talk about planets made by nature.
Once an object reaches a sufficient size, gravity will become greater than the individual "weak bonds" that hold molecules together. When that happens, these bonds will fail and the individual molecules of matter will, each one, try to find their closest spot to the gravitational centre.
THAT is why planets are spherical. That is why stars are spherical.
It is the same idea that determines that liquids (like water) tend to be "level". Because water molecules "slip" over each other, a lake surface tends to be level. You do not see a lake "with a slope".
Yes, there are lakes that can have a tidal slope, but that is so small (lake Superior, in Canada/USA can have a slope of "up to" 4 inches (10 cm) from one end to the other). The tidal effect is called a "mutinodal seiche" (partly caused by tidal effect and by the shape of the shores) -- and was a Command Certificate exam question in 1978 -- at the time, ship captains were expected to understand that kind of stuff.
Along the same line, planets do not need to be "perfectly" round. Earth, for example, is an oblate spheroid: it "looks like a sphere" but has a slightly bigger diameter at the equator than it has from pole to pole.
The Sun ( a slow rotator) is very close to a perfect sphere.