You’re leaving out what happened after Jill Stein lost her bid in 2016: many (not most, not even a lot, but a few) of their positions were adopted by the DNC the following cycle (aspects of the green new deal, weed legalization, even Biden’s student debt relief could partially be attributed to their free tuition position). Third parties (and third party candidates caucusing with the establishment parties) help move the overton window by forcing debate and consideration, even if it doesn’t end up moving the window much.
I’ll even go out on a limb and say that you’re mostly right: electoral politics generally does fuck-all to move the needle for more progressive governance, and even in the few instances where progressives eek out a few concessions they’re usually compromised to all hell and are no longer effective by the time they’re implemented. But saying that voting third party ‘does nothing’ is like denying the existence of gravity. Organized resistance to establishment politics -no matter what form that takes- is possibly the only way to move the needle when it comes to the two party system aside from holding primaries, but you might have noticed that didn’t really happen this year for the democrats. Third parties represent a threat to establishment parties, and because of that threat they have some amount of leverage and bargaining power.
I’ll be charitable again and float the possibility that you do actually see these benefits (small they may be) to third party activities, but you may just think that those small benefits aren’t worth the cost or risk. I’ll even concede that those concerns are fair and justified (for those who still see value in the democratic party), but denying the political reality of those organizations is more than a little defeatist (or wishful thinking, depending on your leaning).
*Edit: and if you’ve still decided you don’t understand those dynamics, then I’ll float the question: what do you think is an effective way to push the needle left?