They're well aware of that slant, and try their damndest to correct for it. But apparently they just don't have a good model of how large or how volatile that propensity attribute is, and how it relates to political leanings.
I was polled this year. One of the first questions I was asked is if I was answering the call on a landline or on a cell phone. They weight those differently. Also of course my basic demographic info (wage, race, marital status, education level, home ownership status, etc.)
I think your whole comment is correct, but instead of not accounting for the propensity factor, they do... just badly :)
I have no idea what the solution is to this. Maybe there isn't one, and instead we can get a silver-lining effect: If people trust the results of polls less, they're hopefully less likely to think the results are preordained. More voter turnout?
In particular, Trump was extremely critical of polls and mail-in voting, and we have pretty much confirmed that his statements caused his supporters to avoid mail-in voting, so it's not too much of a leap to suggest the same thing happened with polls. Notably, this is a different effect from 2016, where IIRC Trump appealed to "non-likely" voters whose responses were inappropriately discounted.
It is a solution that has its own problems, but mandatory voting would solve this problem. The polling failures are primarily in trying to weight the sample that you manage to capture in a way that reflects the actual turnout on election day. If everyone is forced to vote then you eliminate the RV/LV problem and pollsters would only need to make sure their sample was representative of the population at large.
https://news.ycombinator.com/item?id=23548802
https://news.ycombinator.com/item?id=20141336
https://news.ycombinator.com/item?id=20117157
https://news.ycombinator.com/item?id=19751970
https://news.ycombinator.com/item?id=19743834
Are the Uncle Police OK with silent protest? http://v6y.net/unclepolice.mp4
But, what the pollsters got crazy wrong, was Trumps polling numbers, by wide margins, far outside the margin of error.
I think they got it wrong this time too, and I think it all comes down to their methodology for actually getting a random sample of voters. Many polls are still married to live interviews and also to landline contact, and also tied to live interview polling - I think there is a partisan slant that they are not accounting for that includes a "propensity to answer a polling survey in the first place"