They haven't been legally challenged yet. Therefore, they don't want to unilaterally open any cans of worms.
That's what I figure.
Immediately after the elections, they are always start to move a lot quicker to remove them from any ROW, but it's been a few months since the election and I still see a few signs in people's yards. 99% of them are for Trump, and a few have been there since the 2016 election.