Many 'poor' people are poor because of spending habits and lack of saving discipline, and it's a hall mark of the modern poor in America to drive a car you can't afford, own an expensive phone, have a big TV, etc. Whether buying it on credit, or buying with money that could/should have been used to pay off debt, save fore retirement etc, it's trivial to buy something like an iPhone for almost anyone in America.
iPhones are BELOVED by the poor as a "rich" status symbol, for all the same reasons why you think the poor don't own them.
P.S. you can get a brand new iphone for like $30/mo from any major seller. An 18 year old with their first fast food job can buy a big shiny iPhone today.
P.P.S most of the 'upper middle class' I'm familiar with doesn't do iPhones and especially not brand new ones, they do budget phones or cheaper android devices because you don't get wealthy spending $1000 every 2 years on a device that costs $275 to manufacture.