What would make my computer shutdown for no apparent reason, sometimes as often as once a day?

Generally this is due to heat. Computers get very dusty and the fans get filed with dust and the vents get clogged. Luckily this is a easy fix if you have have control over your hands. I would call someone to come out and clean it.

There are little temperature sensors inside the computer and will shut it down if it gets so hot that it might start damaging parts.

Does it go through the proper shutdown routine or does it just shut off immediately?
In the latter case, you should check that the fans and coolers are free of dust. If they are covered with dust, clean them.
Computers have an automatic shutdown feature in case of overheating as a safety measure.

Just shuts down immediately, I had the fans cleaned in the past because that was suspected but nothing changed unfortunately. I will get it checked again though

Check if any of the capacitors (on the mainboard and/or power supply unit) are bulging, e.g. look like this
If they do, the entire affected component must be replaced.

I had a laptop that was doing this (an Acer). I contacted Acer but they were no help, so I took it to Best Buy for their techie people to check out. It was a fan even though it was sort of working. They fixed it and I gave it to one of my kids. It has been almost 2 years and no more problems.

my computer used to shut down with no signs. then i bought another wireless keyboard and it worked fine. maybe you have something wrong of your hardwares.