Early civilizations generally do. A mixture of reasons, but two of the big ones are the ability to retain labor consistently and the ability to absorb defeated enemies.

In societies which are generally operating on a subsistence level, it is important that labor, especially agricultural labor, be constrained in some way or form to ensure that the harvest is sown and reaped. Slavery is not the only way to do this - but regulation of contractual obligations in early civilizations can be sparse, especially when the central state itself is not particularly expansive. Serfdom, or corvee, or other forms of forced labor, are also common for this reason. As methods and laws for regulating labor become stronger, and the central state becomes more established, the need for slavery or serfdom in this sense tends to die off bit by bit.

The other part is that early civilizations are often based around comparatively small ethnicities commanding much, much more vast populations of unlike peoples. When these 'unlike' peoples are cooperative, everything is fine - and such 'cooperative' peoples often end up, over the centuries, being absorbed into the core ethnicity. When these 'unlike' peoples are uncooperative, methods are devised to ensure that they don't up and turn on the early civilization - and one of those methods is depopulation of the effective economically and militarily productive population (through slavery).

Slavery and other forms of forced labor are not a 'mandatory' stage of civilization or anything, but they provide a 'simple' and easily thought of solution to common problems faced by early civilizations.

No. Fortunately slavery is outlawed in every single country on earth now. There's still quite a bit of forced labor and exploitation left, but even that usually does not stoop to the level where people are treated as tradable property, so calling it slavery is a stretch. Humanity has come a long way in that regard.