The gains from selling a house are taxable. In addition, last I checked, 6% of the sale price is split between the buying and selling realtor. But the biggest factor here in California is the controversial Proposition 13. The assessed value of the property for tax purposes can only increase by 2% per year. Since real estate has appreciated in value much faster than 2%, an elderly couple like my parents are only paying about 1/10 of the normal tax rate.
The house my parents bought in 1975 is worth $4 million (according to Zillow) because it is in a prime Palo Alto location. In their case, assuming the house is actually worth $4M, they will make a tidy profit. They could buy a downsized house, but only if they move out of Palo Alto. And their tax bill would sky-rocket.
A better example might my house, which I have owned for 22 years, valued at a "mere" $1.4M. Taxes at market rate would be $12,320. Assuming I bought it in 1975, my yearly tax bill would be a mere $1,232. The first problem, small houses in the Bay Area are still expensive. But suppose I found something for $700K, my tax bill would go from $1,232 to $6,160. OK, maybe I move somewhere cheap, and get a house for $350K, I'm still probably paying more than 2x my current tax rate.
Add to that the enormous stress involved with buying a house, I'm not planning on selling, ever.