Why do light fixtures, dimmer switches, etc. use stranded wire?

Because lights need comparatively low amperage, and are typically located in a protected (indoors) area.

Solid wire is stronger and more reliable in some ways (e.g. corrosion resistant) than stranded wire, but stranded wire is generally easier to work with.

Because it bends easier and even when bent to extremes it normally doesn't break or malfunction. Single strand gets nicks and kinks when bent and just a lot harder for your average person to deal with.

You have to think sometimes you are shoving wires where ever when installing a light fixture or fan. A home owner may not be comfortable pressing as hard as you can with single strand - or more importantly when they do press hard to get the wires into the box they could possibly make their connection faulty producing problems or dangers down the road.

I agree from an electrician point of view - I rather run the single strand in my house to a solid wire because I know how to twist it properly and I know the connection will be good. However the people on this site in the same boat are 2% of the population.

The Lutron dimmer switches are a good example - these are sold to home owners as upgrades. Sure electricians install these but Lutron isn't worried about the electricians - they will make do with whatever. These dimmers will fill up a small outlet box and with possibly 8 other wires in the box you are trying to make the homeowner swapping this thing out have an easier install. It does drive me nuts though when they only have like 1/2" of the stranded bare and you have to cut more insulation out to get it to catch right with solid - connecting stranded smaller wires to solid is an art.

That's only a problem if your wire-nut technique is bad.

Most people's technique is terrible when they start. Where people depart is that some refuse to learn, and do not test their work, or think, or refine their technique. They simply tape the wires onto the nuts to keep them from falling apart. This only hides the symptom; the root problem is a poor electrical connection, which results in overheating, arcing, and a meltdown or fire.

Just don't be that person and you'll be fine.