Why does home ownership on its own matter? Net worth is inclusive of housing and assets and debt. And net worth is a direct measure of the wealth that is being built.
Because homes are pretty much the only asset a millenial would have at that time that would have grown over time. a 08-9 graducate wouldn't really have much money to spar for stocks unless they made really lucky bets or happened to mine a fewbitcoin they forgot about.
401k and IRAs is where millennials should have their stocks and those have done very well over the years. There is little point in stocks elsewhere (unless you are very rich) since stocks are for long term investments and those two cover the retirement needs of nearly everyone (except the very rich), and there are few other savings needs people might have that stocks qualify for.
Remember you won't live forever (at least not to current medical knowledge, you can bet otherwise if you want), and you can't take it with you (according to most religions). Thus once you have retirement covered and emergency savings you should be spending everything you earn. You should have enough money left at the end of the month to afford the things you buy at the end of the month, but there is no point in any more, enjoy life with what you earn. (donating to charity counts as enjoying life!)
Joe is on the hook for his mortgage and so his monthly cashflow must be higher to afford to live. However his net worth is larger and so he is richer.
This is why middle class people often feel poor than those who are poor: they make more money and have more in total, but they also have large bills and if something happens those bills will catch up with them fast.
Net worth isn't equity - mortgage + 401k. Net worth is assets - liabilities. Equity is not an asset; the house is. So is the 401k. The mortgage is a liability. So Joe's net worth is 100k (IRA) + 300k (value of house) - 200k (mortgage) = 200k positive net worth.
(Another way of thinking about equity, specifically, is it is the real estate contribution to net worth, because it is what is left when you subtract the real estate liability (mortgage) from the real estate asset (value of house). That's why you shouldn't subtract the mortgage from the equity: equity is what's left after you've already subtracted the mortgage.)
(Edit: Adjusted sign in first equation to subtract mortgage. It's probably more technically accurate to keep it as addition and consider the mortgage to be a negative value, but I believe it's more straightforward and intuitive for most people as it is now represented.)
As a thought experiment would you not feel (much) poorer if houses suddenly cost >$5 million tomorrow and you didn't own one yet? Even if everything else cost the same? Even if everything else cost the same and your net worth went up $100k?
Lots of research shows about a 8-10% gap, that only at very specific ages finally achieved parity.
The consequence of this is a difference in wealth building, economic security, and family planning for millions.