number go up, number go down. Can't explain that. - Marx

I don't have enough energy/deep lore to go into it but the stock market is all speculative bullshit and is equal parts behind-the-scenes manipulation/corruption, computers jerking each other off all day, and a giant roulette table that Patrick Bateman does coke off of to celebrate Nestle still being allowed to use slaves

Inflation is a real measurable metric on the rate of change of the value of a currency/price of goods while the value of a share in the stock market/market cap of a company depends entirely on supply and demand

You may find Marx's writing on fictitious capital useful for your inquiries https://www.marxists.org/archive/marx/works/1894-c3/ch25.htm

Because there are other sectors of the economy that often usually lag behind inflation.

Wanna guess which ones?