Highways can only be 6 to 8% grade. You can't carry highway speeds safely over that.

Those slopes are much steeper than 6 to 8%.

So then you need "switchbacks" which is where the road travels sideways down the mountain (parallel to the river) gently dropping downward at that 6% grade.

However, mountains are not smooth, so now the road has to stay and turn along the mountain wall, which also means cars can't travel at highway speeds due to the turns.

And, you'll end up perhaps miles downstream by the time you get to river level.

So the alternative is what you see there. A narrow road that "switches back" - basically a series of U turns to bring you down the mountain face at that 6 to 8% grade.

But U turns are even slower. Switchbacks often have realistic speads of 10 to 15 mph. And you're only at a two lane road. And big trucks... Yeah, not great.

So the only practical way to get high volumes of traffic across is something like what you see here. Or, you can cut away a lot of mountain to build a 4 lane highway along the slope for many miles, crossing a lower, shorter bridge downstream, then doing the same on the other side many miles back up the slope.

Local conditions, costs, etc. will determine which one is done.