Depends on how you define "stupid". In many cases you could say "unaware" or "forgetful" instead of "stupid". And mind, I did not say "stupid". That's a word you threw into this.

As to why they still vote like they do:

(1) Because every 4 years the mainstream party promise to solve some problems, netting votes of the young, gullible, or inexperienced voters.

(2) Promise pension increase, netting votes of the ever increasing amount of old people, most who don't care about who or what comes after them.

(3) People are too busy, burned out, or lazy (we still have it too good here, it seems), and cannot be arsed to inform themselves before elections. We also have tons of people, who are truly learning-resistant, right out of school or "university".

(4) People think, that SPD, CDU, Greens, etc. are the only way to stop AfD.

I think we are not very high on the democratic-ability scale. Yes, we vote every 4 years, but it is more like collectively we don't really care enough to inform ourselves properly and just check a few boxes, because we want to tell ourselves, that at least we did vote and that we are fine democratic citizen.

And look, I myself am not frequently reading all parties' positions. And I myself inform myself more shortly before an election, rather than all the time. But I do have a feeling for corruption and I don't always forget scandals that happen, when the next engagement-optimized news headline comes in. I still remember Rezo's "Zerstoerung der CDU" video. I remember reading those abgeordnetenwatch newsletters about the lobby register. Or the foodwatch newsletters about Kloeckner and the Lebensmittelampel. That's why I will not vote for mainstream-promise-a-lot-but-no-delivery parties and retired people clubs. And what more I do, is to use the Wahlomat, and actually check which party's position aligns most with my own.

I don't do too much either, but most people do way less to inform themselves. They just check a box out of habit. Why they vote for CDU/SPD? Because that's what they always did. It's real friggin dumb.

I'm not sure what factual basis you have for your optimism. I have plenty for my pessimism about human nature and our ability to competently govern ourselves as well as our general moral fortitude. We've gotten this far because we've been playing life on easy mode with a ridiculously nurturing planet and practically unlimited energy available under the ground, but we aren't smart enough or forward thinking enough to take on the minor pain necessary to avoid the catastrophe we are lumbering towards. We are short-sighted, selfish, self-important, and we haven't earned the regard in which we hold ourselves.

Regarding your specific point about using recent context to inform political opinions, if you spend any amount of time listening to the opinions of people online you'll find that not only do they fail to accurately recall past events, but they don't bother to research what actually happened, and when they do they fail at anything but the most superficial political analysis.