Bear Stearns and Lehman Brothers were giant investment banks back in the day, but they weren't commercial banks so they didn't advertise to attract customers like the banks who might look for customers to open checking or savings accounts, or small retail brokerage accounts. Their clients/customers were corporations, large institutional investors (insurance companies, mutual funds, university endowments), other investment banks and private equity firms, high net worth investors (those with many millions to invest)... these aren't clients you advertise to on a billboard or magazine ad. Maybe you'd see ads in the Wall St. Journal or Forbes magazine. Certainly you'd see many mentions of them in such publications.

I grew up in an upper middle class area in the '80's-'90's, lawyer dad, lots of lawyer, doctor, trader types among my friends' parents and my parents' friends... I knew of Bear Stearns and Lehman even as a kid because we knew people who worked for them, my dad did work with them (he was an in-house attorney, often doing corporate acquisitions with investment bank involvement). I had friends who had internships with them during college (I myself worked at a portfolio management firm who often managed 7-8 figure accounts held by those firms), or even got jobs with them after college (one of my best friends growing up was an associate at Lehman when it went bust).