We have thousands of them remaining today. And usually buried in sand or in the ocean. That would be too expensive to remove because it's a blockhaus (quite sturdy) and the sand + salt ruin the tools and engines used to destroy them.

And more importantly: it's part of our history. We still keep our old castles so why not keep that. It's a good reminder too.

Here a map of more than 2000 remaining blockhaus : https://www.cestenfrance.fr/cartographie-du-mur-de-latlantique/