While it is true that a slim majority of Americans have some stock, the vast majority of that is in retirement plans, not brokerage accounts.
Most trading volume is algorithmic, so it only takes a small number of real humans doing unusual trading activity for markets to move.
I've phrased it like this because I don't have expertise.
If most capital is in rarely-traded funds (I think 90-95% are passive?) then by definition those last 10%-5% active investors are going to set the market, at least in the general day to day.
I think people have rushed into stocks because they think that that's the safe haven for their money against inflation as they think the Fed is "printing money" and causing inflation, but the data is showing deflation. If deflation continues, bonds would be the safer place to be.
In particular, virtually every single food item at the grocery store is more expensive than it was in March. Am I the only one who has experienced this? It seems like basic staples (milk, eggs, flour) are substantially more expensive than a few months ago.
That's technically correct, though of course does not feel like it for many people. To the average consumer, a "phone that can play today's games" is the same measurement today as it is next year, and if prices don't change, they might not consider that "quality increase".
Regular costs $3.5/gal
But your other point is probably more correct - supply/demand swings in the short term affect the prices you see more than deflation/inflation.
Technically a gallon of milk should go up in price like 6-10 cents a year, which you might not notice.
Most employed Americans are still paying into retirement and mostly dumping cash into index funds on the bet that in 20-40 years the market will be up. Private investors and hedge funds are watching the Fed buy up trillions in bad bets, might as well make more bets with a feeling of lower risk. Even Hertz is gaming the system at a time when it's still very uncertain how the US will cope with the pandemic long term.
However, we are seeing a drop in inflation not quite to deflation but down like 3/4 from above 2% to .65% for recent months: https://inflationdata.com/Inflation/Inflation_Rate/CurrentIn...
This my be due to oil price drops in the past quarter with prices going negative at one point. But generally speaking we are not seeing excess inflation, quite the opposite. I think we are going to see a massive drop off in Q3, especially based on recent weak Q2 earnings reports and likelihood of the US not coping well at all.