While quicksand in real life can be dangerous under unlucky circumstances, fiction generally tends to exaggerate its properties, and ironically tends to ignore the actual factors that can make it a life threat.
It is known that quicksand has a higher density than the human body, so it by itself cannot suck anyone or anything downward. If you barely move, then you'll stay afloat because your torso acts as a buoy of sorts thanks to the lungs. The difficulty one has in escaping from it comes from its extreme viscosity, so it's important not to move too hastily to prevent fatigue.
You could sink if you are weighed down by something you can't remove... but that would happen even in boring old regular water. Indeed, real quicksand requires water, and writers are traditionally unfettered by such technicalities. Fictional quicksand tends to be placed in the desert, or away from a river or any apparent source of water; although a hidden spring could in theory create quicksand in surprising places. In live-action, budget and set-design constraints sometimes lead to a "quicksand pit" barely large enough to hold the actor. Technically speaking, desert quicksand is hand-waved as being very loose sand that looks normal, but is incapable of supporting a large amount of weight.
Although not always strictly "sand", tidal flats (mudflats) have silt and mud which is actually closer to the danger portrayed in fiction as quicksand. The tide regularly refreshes the surface and smooth implies solid to people. Viscosity decreases after an initial stress. More to the point, being a tidal flat, it's a case where people actually ARE at risk of their lives, since tides have this funny way of coming back in after going out. There are tidal flats which have seen so many deaths the locals are tired of watching stupid people die, but it's not wholly clear how much that is getting stuck in one specific spot versus getting trapped some distance from dry ground on slow-to-traverse terrain or attempting to wait on an insecure bit of slightly higher ground as the tide comes in around you.
There is also a hazard in nature called a "predator trap", where an animal gets itself hopelessly stuck in thick, sticky mud or tar and its corpse attracts the attention of multiple carnivores, causing them to become trapped and die of exposure as well — sometimes becoming fossil goldmines. (The La Brea Tar Pits in Los Angeles are a well-known example of one of these fossil troves.)
Quicksand's ability to trap things largely relies on its interesting physical properties, being subject to a phenomenon in common with numerous other non-Newtonian fluids called shear thinning.
When at rest, quicksand appears to be very hard, and its viscosity may be such that it can support the weight of lighter objects on it. However, if given sufficient agitation or shock (the margin of which is extremely narrow), the mixture of water and sand/silt becomes increasingly more elastic as it liquefies, and this is the point where objects begin to sink. This is the point where it becomes dangerous to be around quicksand, as once objects have begun to sink, they reach a point where their buoyancy won't bring them down any further, but the surrounding material returns to its rigid state through shear thickening if more force is applied, thus holding things in place and leaving them at the mercy of further danger from the surrounding environment.
In video games, the trope is prominent in Shifting Sand Land levels and is typically combined with the equally-inaccurate Sand Is Water property. As explained in that very page, sand is a non-Newtonian fluid, which means it won't act like water. You can stand on it when it is stationary, for example, but not when it is fluidized.
