The median salary out of college for a plain middle-class job is $45k. Two such earners, packing away 15% for retirement into index funds, mortgaging a $200k home, and not taking on credit card debt will reach $1m household net worth in middle age.
There is nothing special about the millionaire designation and it does not make them rich, just normal people (at least among the college-educated set) who were lucky enough to avoid layoffs/expensive medical conditions and followed ordinary financial planning advice (save 15%, buy a house, have an emergency fund, pay your credit card bill in full every month). Plenty of such people buy BMWs and do lots of expensive travel; it's still within their means.
If you want to understand the behavior of the truly rich business owner/capitalist/old money class, don't pollute your sample with families of teachers, accountants, middle managers, IT department workers, etc.
Also, a Tahoe is $46k. Yet I imagine you wouldn't be pointing to the frugality of the rich if he were driving a $46k BMW.