A lot of us did. Only one car I owned (the first, for nine months) had just an AM radio. But I knew I was replacing it as soon as I had the money saved, and I budgeted for an 8-track deck (it was 1973) and a cartridge-style FM converter.
Remember, though---if you've already paid $75 for a factory AM radio, adding to that is a big swallow. That $75 PTBoardOp cites from 1976 for an AM radio is $373.97 today. The $226 for AM/FM? That's $1,126.89 in today's money. Top-of-the-line iPhone money. Aftermarket was only a little less expensive (unless you went for cheap/bad), speakers were sold separately and installation cost money.
And in the 70s, the adults/parents were products of the Great Depression. My mom thought I was insane to drop $175 (installed) for a stereo in a car I'd just bought with a "perfectly good" AM radio that...yes, I had paid for. I could have ordered either an AM/FM mono or an AM/FM stereo radio in the car, but the prices, as we see above, were nuts. And this model didn't have a factory tape deck option and I wanted to be able to pick my own music sometimes. So I knew I could do better, if only slightly less expensively, with an under-dash Clarion 8-track, four speakers and a cartridge-style FM adaptor.
An interesting thing though: A lot more of us in my hometown of 3,000 people with only one radio station (AM) and no outside over-the-air signals until sundown (there was Los Angeles FM on cable and a 10-watt translator of dubious legality rebroadcasting KKDJ) had tape decks and FM in our cars than did friends of mine in L.A. We were reacting to a lack of choice. But there was still a lot on the AM band in Los Angeles. So people made do with what was the least expensive. Which, if you look at an AM radio as $373 and an AM/FM as $1,100, is pretty understandable. In fact, now, I realize I should have felt ripped off by the AM price.