The majority of incoming calls on landlines now are people trying to scam the callee. [https://www.inc.com/bill-murphy-jr/almost-half-your-phone-ca...]. And the majority of AM radio content now is low-audience long-tail content (or mass-commodified syndicated content, like Rush Limbaugh) that is a medium for pushing low-quality bulk advertising for questionable products.

To expand upon the hypothesis (and this is half-baked and incomplete, so take it with several grains of salt): as technology evolves, people move to more comfortable / more convenient technology. AM is not comfortable or convenient; it's interfered with by too many EM flux sources in the modern world. Landline is not comfortable or convenient for the reasons you noted relative to modern alternatives.

The people who do not move off these technologies are various flavors of captive audiences: people who can't buy FM radios or don't want to adopt new stations / find content in new locations, people who can't use a cellphone, etc. As mainstream content creation leaves these channels, the vacuum is taken over by hucksters trying to take advantage of these captive audiences. The incentives to do so are lack of alternative content and a "softer" target audience (easier to fool, especially since these technologies were once mainstream and trusted so many of their users still believe they are, even after the hucksters have taken them over). I have relatives who still believe "They wouldn't call me if they didn't have business with me; how would they know my number?" And I still have relatives who believe AM news radio is news.