The problem is is that they r not needed in the game, all rivers and streams can be crossed with TMM,AVLB already. The only place u could use it is those 1-2 wide water hexes north of Hamburg and 1 hex=1603meters and the longest pontoon bridge ever build that i found was in 1995 and it was 678 meters long. I c why HPS left them out, theres already other units doing the same job.
I understand what u r saying about pontoons being longer and AVLB not being able to reach across some rivers but in the actual game the AVLB,TMM already do reach across all the rivers and streams.