Why don't direct candidates strategically withdraw to block the AfD?

Apologies if this is a dumb question about German politics and the electoral system, but so far I could not find an answer to it just by googling.

My understanding is that it is in the general interest of every non-AfD party (Union aside) to make sure the AfD has as little of a fraction in the Bundestag as possible. One of the two ways the AfD's fraction grows is by having direct candidates who win constituencies.

Because polls exist, we know in advance the likelihood of a party candidate winning a constituency. If an AfD win is guaranteed, why don't candidates with no change of winning withdraw with the message that voters should vote for someone else in order to prevent the AfD's win?

For example, in the 2021 election results I could find at least 4 constituencies (only looked at Thüringen where I know AfD is strong), where votes cast for the SPD and the Linke candidate combined would be higher than the AfD's winning result. In these cases, with no chance of winning, why doesn't the Linke (just as an example) withdraw and say people should vote for the SPD's candidate, to prevent the AfD taking the seat?

I want to stress that I'm not proposing they do this, obviously it's a can of worms morally and politically. I'm simply curious as to why such strategizing has not developed in German politics.