why are the supply of homes for sale so low?

Most of the people who have died from COVID have been in nursing homes--or were in long-term care facilities. That means they weren't living in homes. Also, many of the deaths were in a family of people who are still living in their home. COVID has not 'vacated' properties, though it has killed a lot of people. Rental markets aren't moving much either right now, because of the eviction moratoriums in many places. People are just staying where they are. 

Also, this really depends on where you're looking. There are some areas where there are tons of homes for sale--and it has really very little to do with the pandemic. What you need to do is look in areas where people are moving OUT.  That's going to be different in every state; some cities are becoming less populated, leaving quite a few homes for sale. Do some research and find out where the market DOES have a lot of home inventory for sale, and concentrate your efforts there. 

I don't know what you are talking about.

I saw a cabin, for sale, for $150,000, and I said to the person who was selling it that was a rip off. I don't how much land is worth. Because I saw many different quotes, for land, but not with cabins. I know what the value of cabins are. The cabin I saw, for $150,000 looked similar to the cabin I saw that was built, for $7,000.

I saw many houses, for sale as well!

Just because someone dies - from covid or anything else - does not mean the other family members living there are going to sell their home. Also, with lockdowns in so many areas, some people hesitate to list their home for sale, concerned about the lack of traffic, open houses, etc, or afraid someone coming through might expose them to the virus. Furthermore, people are fleeing major cities, like NYC & practically anywhere in California, so demand in many smaller communities is very high. So what market are you househunting in? There are a number of factors going on here. 

Only about 1/10 of 1% of the population died of COVID.  And many of them either lived in a nursing home or lived with someone else who didn't die and is still living in their home.

Even if everyone who has died of COVID lived alone, owned the property they lived in, and didn't share their home with anyone else who might want/need to continue living there...

Probate takes months; belongings would need to be cleared (which if family aren't close may not happen quickly due to travel restrictions - see pandemic); so houses can't immediately be sold. 

good think young adults don't grow up and buy houses.

Uncertainty about keeping your job might prevent you wanting to move, I'm surprised I still have mine. Not that many people have really died, it sounds like a lot but spread out all over everywhere, not really and, they were not all homeowners so scale it back again by a lot. On top of that, when a homeowner dies it usually transfers to a relative or someone they left it to who will often not sell it.