A large chunk of the human population has the ability to grow beards, but many people want to ensure that their facial hair is thick and luxurious.
A popular misconception says that shaving beards causes them to grow back thicker, but in reality, this is not exactly the case. Hair in general is tapered, meaning the end is thinner than the part near the root. When people shave their beards, they cut off the thin end.
The hair continues to grow out, but the visible thin hair is being replaced by thicker hair. This gives the appearance that a beard is growing back more thickly than it was before, but a similar effect can be achieved by simply growing out a beard for a few weeks.
Similarly, waxing does not cause hair to grow back differently than it was before. Occasionally, waxing might irritate the skin enough to form a callous, which might lead to thicker hair. This is not typical, and usually waxing will only remove hair temporarily.
Hair might appear thicker due to being darker when it first grows back as well. Hair is normally exposed to sunlight and pollutants, which might change its color. Freshly-grown hair is not impacted by that.